Thanks for your answer!
Indeed it would seem, that the empty fields (e.g. “Editor” in a Journal Article) cause the Problem. So either I put ghost entries in those fields or I import per Reference Type.
I thought that generic is just about that: being able to import several types at once. Was that a mistake?
Greetings
edit: ah, I see, generic requires different field names. I think, with almost 1000 entries, it’s easier to go ref type by ref type. And thanks for the advice with the Carriage Returns.