Hi Alex,
Thanks for your reply.
The original MARC21 file, exported from the Kobli application, is nearly 10 MB in size and contains around 9,000 records.

Since I was having trouble importing it into Dédalo, I opened it using MarcEdit and split it into several smaller files.
This allowed me to import it into Dédalo, but the strange behavior described in this thread occurred.
I tried attaching the first of these files—the one I am attempting to import into Dédalo—to the thread, but I received an "Uploading files of this type is not allowed" error. I also tried compressing it into a ZIP/RAR archive and changing the extension to .txt, but neither worked.
I am pasting the first 5 records here, exactly as they appear in MarcEdit.
=LDR 01134nam a22003011 4500
=001 7028
=003 b7028
=005 20230605091601.0
=007 ta
=008 960722s1969\\sp\\\\\\\\\spa\
=017 \$aA 7-1970$bOficina Depósito Legal Alicante
=035 \$a(OCoLC)432822157
=040 \$aSpMaBN$bspa
=080 \0$a946.733 Villena"12/17"(093.2)
=084 \$aR7028
=100 1\$91$aSoler García, José María$d1905-1996
=245 13$aLa relación de Villena de 1575$htexto impreso$cJosé María Soler García ; edición comentada y apéndice documental$bEdición comentada y Apéndice documental
=250 \$a2º
=260 \$aAlicante$bInstituto de Estudios Alicantinos, Diputación Provincial $c[1969]
=300 \$a619 p., 3 h.$c22 cm
=490 1\$aPublicaciones del Instituto de Estudios Alicantinos$vv. I
=650 \$2embne
=651 \7$aVillena$xHistoria$vFuentes$2embne
=700 1\$91$aSoler García, José María$d1905-1996
=710 2\$947$aInstituto de Estudios Alicantinos
=942 \$2udc$cBK
=999 \$c1$d1
=952 \$p2017-0001$r2017-12-15$40$00$6R7028$91$b031409$10$oR7028$d2017-12-15$70$yBK$a031409
=LDR 01528nam a22003617a 4500
=001 b.5505
=003 b.5505
=005 20190311190126.0
=008 190227b\\\\xxu|||||\00|\0\spa\d
=017 \$aA 471-1976$bOficina Depósito Legal Alicante
=020 \$a8450016878
=035 \$a(OCoLC)5505
=040 \$aSpMaBN$bspa
=080 \0$a903(460.315 Villena)
=080 \0$a904(460.315 Villena)
=080 \0$a94(460.315 Villena)
=084 \$aFV-0020
=084 \$aR5505
=100 1\$91$aSoler García, José María$d1905-1996
=245 10$aVillena$htexto impreso$bprehistoria, historia, monumentos$c[por] José María Soler García
=260 \$aAlicante$bExcma. Diputación Provincial$c1976
=300 \$aIX p., 1 h., 176 p., 2 h.$bil.$c31 cm
=650 \$2embne
=651 \7$aVillena$xRestos arqueológicos $2embne$91643
=651 \7$aVillena$xMonumentos$2embne
=942 \$2udc$cBK
=999 \$c2$d2
=952 \$p2017-0002$r2017-12-26$40$00$6FV0020$9116$b031409$10$oFV-0020$d2017-12-26$70$yBK$a031409
=952 \$p2019-0885$r2019-03-11$40$00$6FV0020$94919$b031409$10$oFV-0020$d2019-03-11$70$yBK$a031409
=952 \$p2019-0886$r2019-03-11$40$00$6FV0020$94920$b031409$10$oFV-0020$d2019-03-11$70$yBK$a031409
=952 \$p2019-0887$r2019-03-11$40$00$6FV0020$94921$b031409$10$oFV-0020$d2019-03-11$70$yBK$a031409
=952 \$p2019-0888$r2019-03-11$40$00$6FV0020$94922$b031409$10$oFV-0020$d2019-03-11$70$yBK$a031409
=952 \$p2019-0889$r2019-03-11$40$00$6FV0020$94923$b031409$10$oFV-0020$d2019-03-11$70$yBK$a031409
=LDR 01186cam a22003011i 4500
=001 bimo0001124901
=003 SpMaBN
=005 20180713101802.0
=008 920725s1973\\sp\\\\\\\\\spa\
=016 7\$abimoBNE19993753569$zIND9200535916$2SpMaBN
=017 \$aA 389-1973$bOficina Depósito Legal Alicante
=020 \$a8450059968
=035 \$a(OCoLC)432559307
=040 \$aSpMaBN$bspa$cA2
=080 \0$a946.74"07"
=084 \$aR99
=100 1\$948$aLlobregat Conesa, Enrique A.$d1941-2003
=245 10$aTeodomiro de Oriola, su vida y su obra$htexto impreso$cEnrique A. Llobregat Conesa
=260 \$aAlicante$bCaja de Ahorros Provincial $c1973
=300 \$a111 p., [3] h., [1] h. de map.$c22 cm
=490 1\$aPublicaciones de la Caja de Ahorros Provincial de la Excma. Diputación de Alicante$v17
=651 \7$aMurcia$xHistoria$yS.VIII$2embne
=710 \$946$aCaja de Ahorros Provincial de Alicante$bObra social y cultural
=810 2\$960$aPublicaciones de la Caja de Ahorros Provincial de la Excma. Diputación de Alicante
=942 \$2udc$cBK
=956 \$a1 2
=999 \$c3$d3
=952 \$p2017-0003$r2017-12-20$40$00$6R99$976$b031409$10$oR99$d2017-12-20$70$yBK$a031409
=LDR 00892cam a22002891 4500
=001 8396293
=003 8396293
=005 20190102190002.0
=007 ta
=008 771018s1955\\xx\\\\\\000\0\ita\
=010 \$a 55042867
=035 \$a(OCoLC)3351188
=040 \$aDLC$cOC$dDLC
=041 0\$aita
=042 \$apremarc
=080 \$a739.1(391)"02/05"
=084 \$aR1
=100 \$92$aBecatti, Giovanni$d1912-1973
=245 10$aOreficerie antiche dalle minoiche alle barbariche$htexto impreso
=246 \$aOrfebres antiguos del minoico a los bárbaros
=260 \$aRoma$bIstituto poligrafico dello Stato$c1955
=300 \$a255 p.$b178 lam.$c30 cm.
=650 \0$93$vOrfebrería$aJoyería$yAntigüedad
=906 \$a7$bcbc$coclcrpl$du$encip$f19$gy-gencatlg
=942 \$2udc$cBK
=999 \$c4$d4
=952 \$p2017-0018$r2017-12-15$40$00$6R1$97$b031409$10$oR1$d2017-12-15$70$yBK$a031409
=LDR 02609nam a22004331 4500
=001 Mimo0000642508
=003 SpMaBN
=005 20180713140002.0
=008 900725s1972\\sp\\\\\\\\\spa\
=016 7\$abimoBNE19971222029$zIND9100059484$2SpMaBN
=017 \$aBI 1410-1972$bOficina Depósito Legal Vizcaya
=035 \$a(OCoLC)433757170
=040 \$aSpMaBN$bspa$cA2
=080 \0$a008(=916.9)
=084 \$aR2
=100 1\$94$aBarandiarán, José Miguel de$d1889-1991
=243 10$aObras
=245 10$aObras completas$htexto impreso
=260 \$aBilbao$bLa Gran Enciclopedia Vasca$c1972
=300 \$a 13 v.$blám. neg. y col., grab.$c23 cm
=505 0\$aI. Diccionario ilustrado de mitología vasca,452 p. II. Eusko-folklore,524 p. III. Ikuska 1 IV. Ikuska 2, 511 p. V. Ikuska 3, 510 p. VI. Ikuska 4, 456 p. VII. Vasconia antigua. Tras las huellas del hombre, 480 p. VIII. Vasconia antigua. Tras las huellas del hombre (II), 455 p. IX. Vasconia antigua. ras las huellas del hombre (III), 504 p. X. Vasconia antigua. Tras las huellas del hombre (IV), 549 p. XI. Vasconia antigua. Tras las huellas del hombre (V), 455 p. XII. Vasconia antigua. Tras las huellas del hombre (VI), 451 p. XII. Vasconia antigua. Tras las huellas del hombre (VII), 504 p.
=650 \7$aVascos$2embne
=653 \$aHistoria
=942 \$2udc$cREF
=956 \$a1 2
=999 \$c5$d5
=952 \$p2017-0004$r2017-12-15$40$00$6R2$98$b031409$10$oR2$d2017-12-15$70$yREF$a031409
=952 \$p2017-0005$r2017-12-26$40$00$6R2$9117$b031409$10$oR2$d2017-12-26$70$yREF$a031409
=952 \$p2017-0006$r2017-12-26$40$00$6R2$9118$b031409$10$oR2$d2017-12-26$70$yREF$a031409
=952 \$p2017-0007$r2017-12-26$40$00$6R2$9119$b031409$10$oR2$d2017-12-26$70$yREF$a031409
=952 \$p2017-0008$r2017-12-26$40$00$6R2$9120$b031409$10$oR2$d2017-12-26$70$yREF$a031409
=952 \$p2017-0009$r2017-12-26$40$00$6R2$9121$b031409$10$oR2$d2017-12-26$70$yREF$a031409
=952 \$p2017-0010$r2017-12-26$40$00$6R2$9122$b031409$10$oR2$d2017-12-26$70$yREF$a031409
=952 \$p2017-0011$r2017-12-26$40$00$6R2$9123$b031409$10$oR2$d2017-12-26$70$yREF$a031409
=952 \$p2017-0012$r2017-12-26$40$00$6R2$9124$b031409$10$oR2$d2017-12-26$70$yREF$a031409
=952 \$p2017-0013$r2017-12-26$40$00$6R2$9125$b031409$10$oR2$d2017-12-26$70$yREF$a031409
=952 \$p2017-0014$r2017-12-26$40$00$6R2$9126$b031409$10$oR2$d2017-12-26$70$yREF$a031409
=952 \$p2017-0015$r2017-12-26$40$00$6R2$9127$b031409$10$oR2$d2017-12-26$70$yREF$a031409
=952 \$p2017-0016$r2017-12-26$40$00$6R2$9128$b031409$10$oR2$d2017-12-26$70$yREF$a031409
And here are the first 5 records in .mrc format:
01134nam a22003011 45000010005000000030006000050050017000110070003000280080041000310170047000720350021001190400016001400800034001560840010001901000046002002450166002462500009004122600081004213000024005024900062005266500010005886510038005987000046006367100042006829420012007249990009007369520087007457028b702820230605091601.0ta960722s1969 sp spa aA 7-1970bOficina Depósito Legal Alicante a(OCoLC)432822157 aSpMaBNbspa 0a946.733 Villena"12/17"(093.2) aR70281 91aSoler García, José Maríad1905-199613aLa relación de Villena de 1575htexto impresocJosé María Soler García ; edición comentada y apéndice documentalbEdición comentada y Apéndice documental a2º aAlicantebInstituto de Estudios Alicantinos, Diputación Provincial c[1969] a619 p., 3 h.c22 cm1 aPublicaciones del Instituto de Estudios Alicantinosvv. I 2embne 7aVillenaxHistoriavFuentes2embne1 91aSoler García, José Maríad1905-19962 947aInstituto de Estudios Alicantinos 2udccBK c1d1 p2017-0001r2017-12-1540006R702891b03140910oR7028d2017-12-1570yBKa03140901528nam a22003617a 4500001000700000003000700007005001700014008004100031017004900072020001500121035001600136040001600152080002500168080002500193080002400218084001200242084001000254100004600264245009600310260005000406300004200456650001000498651004900508651003100557942001200588999000900600952009200609952009300701952009300794952009300887952009300980952009301073b.5505b.550520190311190126.0190227b xxu| |||| 00| 0 spa d aA 471-1976bOficina Depósito Legal Alicante a8450016878 a(OCoLC)5505 aSpMaBNbspa 0a903(460.315 Villena) 0a904(460.315 Villena) 0a94(460.315 Villena) aFV-0020 aR55051 91aSoler García, José Maríad1905-199610aVillenahtexto impresobprehistoria, historia, monumentosc[por] José María Soler García aAlicantebExcma. Diputación Provincialc1976 aIX p., 1 h., 176 p., 2 h.bil.c31 cm 2embne 7aVillenaxRestos arqueológicos 2embne91643 7aVillenaxMonumentos2embne 2udccBK c2d2 p2017-0002r2017-12-2640006FV00209116b03140910oFV-0020d2017-12-2670yBKa031409 p2019-0885r2019-03-1140006FV002094919b03140910oFV-0020d2019-03-1170yBKa031409 p2019-0886r2019-03-1140006FV002094920b03140910oFV-0020d2019-03-1170yBKa031409 p2019-0887r2019-03-1140006FV002094921b03140910oFV-0020d2019-03-1170yBKa031409 p2019-0888r2019-03-1140006FV002094922b03140910oFV-0020d2019-03-1170yBKa031409 p2019-0889r2019-03-1140006FV002094923b03140910oFV-0020d2019-03-1170yBKa03140901186cam a22003011i 4500001001500000003000700015005001700022008004100039016004600080017004900126020001500175035002100190040002000211080001500231084000800246100004900254245008700303260004800390300004200438490009200480651003600572710007100608810009200679942001200771956000800783999000900791952008400800bimo0001124901SpMaBN20180713101802.0920725s1973 sp spa 7 abimoBNE19993753569zIND92005359162SpMaBN aA 389-1973bOficina Depósito Legal Alicante a8450059968 a(OCoLC)432559307 aSpMaBNbspacA2 0a946.74"07" aR991 948aLlobregat Conesa, Enrique A.d1941-2003 10aTeodomiro de Oriola, su vida y su obrahtexto impresocEnrique A. Llobregat Conesa aAlicantebCaja de Ahorros Provincial c1973 a111 p., [3] h., [1] h. de map.c22 cm1 aPublicaciones de la Caja de Ahorros Provincial de la Excma. Diputación de Alicantev17 7aMurciaxHistoriayS.VIII2embne 946aCaja de Ahorros Provincial de AlicantebObra social y cultural2 960aPublicaciones de la Caja de Ahorros Provincial de la Excma. Diputación de Alicante 2udccBK a1 2 c3d3 p2017-0003r2017-12-2040006R99976b03140910oR99d2017-12-2070yBKa03140900892cam a22002891 45000010008000000030008000080050017000160070003000330080041000360100017000770350019000940400017001130410008001300420012001380800023001500840007001731000036001802450069002162460050002852600049003353000029003846500042004139060045004559420012005009990009005129520081005218396293839629320190102190002.0ta771018s1955 xx 000 0 ita a 55042867 a(OCoLC)3351188 aDLCcOCdDLC0 aita apremarc a739.1(391)"02/05" aR1 92aBecatti, Giovannid1912-197310aOreficerie antiche dalle minoiche alle barbarichehtexto impreso aOrfebres antiguos del minoico a los bárbaros aRomabIstituto poligrafico dello Statoc1955 a255 p.b178 lam.c30 cm. 093vOrfebreríaaJoyeríayAntigüedad a7bcbccoclcrplduencipf19gy-gencatlg 2udccBK c4d4 p2017-0018r2017-12-1540006R197b03140910oR1d2017-12-1570yBKa03140902609nam a22004331 4500001001500000003000700015005001700022008004100039016004600080017005000126035002100176040002000197080001600217084000700233100004800240243001000288245003500298260004500333300004400378505060200422650001801024653001301042942001301055956000801068999000901076952008201085952008401167952008401251952008401335952008401419952008401503952008401587952008401671952008401755952008401839952008401923952008402007952008402091Mimo0000642508SpMaBN20180713140002.0900725s1972 sp spa 7 abimoBNE19971222029zIND91000594842SpMaBN aBI 1410-1972bOficina Depósito Legal Vizcaya a(OCoLC)433757170 aSpMaBNbspacA2 0a008(=916.9) aR21 94aBarandiarán, José Miguel ded1889-199110aObras10aObras completashtexto impreso aBilbaobLa Gran Enciclopedia Vascac1972 a 13 v.blám. neg. y col., grab.c23 cm0 aI. Diccionario ilustrado de mitología vasca,452 p.
II. Eusko-folklore,524 p.
III. Ikuska 1 IV. Ikuska 2, 511 p. V. Ikuska 3, 510 p.
VI. Ikuska 4, 456 p.
VII. Vasconia antigua. Tras las huellas del hombre, 480 p.
VIII. Vasconia antigua. Tras las huellas del hombre (II), 455 p.
IX. Vasconia antigua. ras las huellas del hombre (III), 504 p.
X. Vasconia antigua. Tras las huellas del hombre (IV), 549 p.
XI. Vasconia antigua. Tras las huellas del hombre (V), 455 p.
XII. Vasconia antigua. Tras las huellas del hombre (VI), 451 p.
XII. Vasconia antigua. Tras las huellas del hombre (VII), 504 p. 7aVascos2embne aHistoria 2udccREF a1 2 c5d5 p2017-0004r2017-12-1540006R298b03140910oR2d2017-12-1570yREFa031409 p2017-0005r2017-12-2640006R29117b03140910oR2d2017-12-2670yREFa031409 p2017-0006r2017-12-2640006R29118b03140910oR2d2017-12-2670yREFa031409 p2017-0007r2017-12-2640006R29119b03140910oR2d2017-12-2670yREFa031409 p2017-0008r2017-12-2640006R29120b03140910oR2d2017-12-2670yREFa031409 p2017-0009r2017-12-2640006R29121b03140910oR2d2017-12-2670yREFa031409 p2017-0010r2017-12-2640006R29122b03140910oR2d2017-12-2670yREFa031409 p2017-0011r2017-12-2640006R29123b03140910oR2d2017-12-2670yREFa031409 p2017-0012r2017-12-2640006R29124b03140910oR2d2017-12-2670yREFa031409 p2017-0013r2017-12-2640006R29125b03140910oR2d2017-12-2670yREFa031409 p2017-0014r2017-12-2640006R29126b03140910oR2d2017-12-2670yREFa031409 p2017-0015r2017-12-2640006R29127b03140910oR2d2017-12-2670yREFa031409 p2017-0016r2017-12-2640006R29128b03140910oR2d2017-12-2670yREFa031409
Many thanks.