[Koha] Importing staged records stops at #53849 (Koha 3.00.01.005)

Dear List, I am struggling with the webinterface import of staged MARC records: The import seems to stop invariably at rec #53849. Oddly enough, there seems to be no broken MARC record near this number. I can indivually import all the records around it including #53849, and even if I delete these five from the import file, so that they are replaced by other records, it just happens again: no importing beyond MARC #53849. Any ideas? Best regards, Marc