Another piece of information ...
During the last couple of weeks I was testing the conversion from Classic to Threads on the same forum with a few selected users. At that time, the Today's Birthday Island was working.
Last night I made the conversion on a production basis.
I dropped the SQL database. Redefined it and ran createtables.php successfully. Then I ran the classic_import script successfully. Then I went in and tweaked a few CP settings.
I did not delete the ubbthreads directory and completely start over.
