I have tried to check whats going wrong and googled a little bit around. I have inspect the cookies with chrome and saw, that a (in some posts mentioned) cookie named „key“ was never set.
After that, I have start to deactivate some of my chrome extensions and viola … login works again … the problematic extension I have found out was
„Simple Adblock“ which I have installed in the last weeks.
Now I’m using again „AdBlock Beta“ and added my webmail domain to the white list filter.
Everything fine again 🙂