It has been a long time for me using classic.
But have you tried the following in the control panel:
Rebuild & Reindex (forum stat files, search index, member stat files)
UBB.classic™ uses a series of files ("metadata" files) to store frequently requested data about members and topics within a forum. From time to time, the information stored in these files may drift out of sync, resulting in occasional errors.
You should not need to rebuild these data files unless directed to by an error message or Groupee support.
To rebuild the metadata files, click Rebuild & Reindex, and choose the specific files to be rebuilt.
This same mechanism can be used to rebuild the search index (see the Rebuild Search Index tab on the same page).
I would suggest you make a backup before you use that option.