OK, iｴve found out more:

i deleted all my cookies. so i was definitly logged out.
When loggin in again manually, the same problem occured again. The Site is Relogging me in al the time, when i cange to forum-pages or Calendar-entrys or ehrni try to logout.

Im admin of a second Xoops site, which is a simple clone of www.khazgoroth.de. The strange thing is, that on this second site NO problems occur. everything works very well there.
Im totally confused. Why is the one site working as alwasy, and the other site making this trouble since this evening? The only big difference between the 2 Site is the member-number. The Still-Working-site has about 20 members, the Not-Working-Site hast now 199 members.

Oh my god. i need help as soon as possible.
Thx, Dan
Mail@daniel-cron.de
icq 83769473