Thats a tough call since there are only a few people that visit here that still use classic.
But anyway I recall there was a file in the forums folder that held all of the forum data, not the posts which are in a separate folder but one for all of them.
Probably one of the var files which you should not over write if existing during a upgrade.
You said you had 4 forums so you should have four folders ,forum1 ,fourm2,forum3 and forum4.
But there is another file that I believe was just above them in the path to them.
I would guess you have a corrupted file.

But it may be better to just restore your old version in another folder check that it works then upgrade again. if that works then move classic to the old location and update your var files per the documentation for moving.

Blue Man Group
There is no such thing as stupid questions. Just stupid answers