[Amenadiel]

<font size="3">EDIT: 5 hours later</font>

I found out that the problem was in the plugin showthread_getinfo

For some odd reason, the line

if ($request_uri != $supposed_uri)

was always true, so even the SEOed URL were being 301 redirected.

I inserted a few debug lines and found out that $request_uri resulted in an empty string... WTF, perhaps that's because I have register_globals off.

Anyway, I replaced the $request_uri for the original command $_SERVER['REQUEST_URI'] and everything works fine now.