Ok, since the issues I brought up on my last post were also happening in other parts of the site (for instance, in the Control Panel, while choosing an avatar, users could not choose any page other than 2 - we've got lots of avatars), he would get an URL such as the one described on the past post.
I've just uninstalled all the parts of this hack that deal with page navigation and, of course, everything is working fine again. Of course search engines won't be indexing any pages other than page 1 of all of my threads, but at least everything is working.
If anyone got these problems and/or wants to try to fix them, I'll try to help.
Just thought I'd post my experiences here.