Yes. The only thing that also changed was the IP address for the mailserver, which has been updated.
Forums that existed and had posts are fine.
Existing forums that had 0 posts caused the error when you tried to open the forum.
If I rename the cache directory, I can then open the forum without the message. (Last time though, if I then posted a message, it repeated the error, however, unbeknown to me at the time, it DID actually add the post to the forum.)
I've just renamed the cache folder so it will create a new one, and then I am going to rebuild the forum stats before attempting to post, and see if that helps.