Against all critics, first class support! :up:
I'll try what you adviced.
Sorry to bother, but I got something else.

I got the SEO URLs activated and navigation through my forum works fine, no problems.
But on forumhome, when I'm not using my home language (german), the flags (language links) look like this:
(choosen language english)
spanish: mysite.de/es/?language=en
french: mysite.de/fr/?language=en
back to german: mysite.de/?language=en
So I can't change a language on forumhome.
Within the threads and forumdisplay, also no problems.
Is this my misconfiguration or a bug?