I tried changing the encoding for the source database to latin_swedish_ci in the import configuration, since it seemed to be looking for UTF-8.
Ran the import again. Same errors.
It always occurs at the same point, around post 2,948,173 or 71.12% of the table. That's not necessarily the post ID of course, so it doesn't really help but it's consistent. phpMyAdmin finds no errors in the table (I ran repair three times as well).
I'm pretty much stumped now. Don't know what else to try.