When did this start happening, maybe after a new plugin was installed? If so deactivate that plugin and try again.
Have you tried removing the forum plugin after backing up the database. Then reinstall the forum plugin and see if a clean forum does the same thing. If the forum works okay, restore the database and try the forum again, if it fails, they is probably a character set problem.
How old is your site? Do you have lots of data in your forum tables?
Have you made any changes to your database structure, done any edits to your fiels, installed a new plugin recently?
When does this happen, can you open the forum? Does it load?