It was time to login to SourceForge again but upon providing my credentials, all I got was:

Error 403
We're sorry, but we could not fulfill your request for /account/login.php on this server.
You do not have permission to access this server.
Your technical support key is: 43ae-fa75-17f4-e8c8

Please contact ... and be sure to provide the technical support key shown above
and as much information as you can so we can resolve the problem.

Well, I really needed to login so I contacted the support guys. Although they responded within minutes(!), they were not really sure why I do not have permission to access this server. Something about my useragent, but then it could be my IP address as well. After a few exchanges it became clear that it was really my useragent what's being blocked. As I've modified general.useragent.override this must've been catched by some of their blacklists. To be specific, the following string was blocked:

     Mozilla/5.0 (X11; U; Linux; en-US; rv:1.0) Gecko/25250101

While the following allowed me to login:

     Mozilla/5.0 (X11; U; Linux; en-US; rv:1.0) Gecko/20050316

Now, the only tow questions left open are:

~~What kind of blacklist logic filters on useragent strings?~~
Why can't they put a nice desciptive explanation in the error message instead of this tedious support key?

Update: their helpdesk turned out to be more helpful as I expected and they provided me with a few details. Apparently it was the BadBehaviour filter which refused my login with the weird useragent.