Just a coincidence: this morning the same client is doing an export import right now and experiences many errors:
Failed to import a record of type MailReadVc pos 244704
....
failed to import a record of type RLinkVc 3680555
Warning: keys are not unique(1) for register RLinkVc key MainKey record Record Linking
...


In a previous import they had much less of these errors.
They haven't changed a version. They are on older 8.4

Maybe you have the same errors?