I'm in the process of migrating from a older community installation to the current supported version. When I copy a mailbox and then open webmail I'll get this error:

I tried gromox-mbck, which outputs this:
== exchange.sqlite3 ==
ck_allocated_eids: [0 issues]
ck_indices_present:
I tried export, but then import fails. What options do I have to restore the mailbox/database properly?