Hmm...I'm not able to reproduce this.
When you say you can't log into your forums, do you mean you just can't access your site? Or do you mean you can access your site, but not the forum part of your site.
we could all get intoo the site but not the forums.
we tried to merge a post and each time we tried to get to forums we al lgot a fatal error but dont know if you done anythine but it working again.
if you havnt done nothing when you got tiem check php for thread when trying to merge etc.