Every nite around the same time until morning the next day my site goes down for logged in accounts and throws this error.
have you tried repairing the referred table...
REPAIR TABLE `forum_users`;
PS: is it correct that your tables have a "forum" prefix and not mybb.. also contact your host to make sure that the db disk is not full/exceeding its limit..
So im a java guy and dont know much about webhosting. All other features of the site work. Just not the forums. i dont even know to how repair the table, and the way you said it makes it seem like i should.
to repair your table, you need to do it via phpMyAdmin. you should be able to access that on your cPanel.