Hi Radek,
yes you are right, that was needed
I has to comment out the line: # RewriteBase /
and also my Apache httpd.conf wasn't set right
In the FAQ I have found a Thread about how to test for that:
http://processwire.com/talk/topic/1-what-is-the-admin-login-url/#entry24096
And yes, I've better should used the search-function before posting this thing
Horst