Yeah!! I have found the problem

Recently i have added a mod_rewrite rule in .htaccess to remove trailing slashes from URL's (due SEO reasons). This was the code:
# Trailing Slash entfernen
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_URI} !^(.*)administrator/
RewriteCond %{REQUEST_URI} !^(.*)component/users
RewriteCond %{REQUEST_URI} (.*)$
RewriteRule ^(.+)/$ http://www.spotcatch.net/$1 [R=301,L]
I think the failure is generated somewhere at "component/users". I added this rule, to be still able to login at my site, cause the URL for the login form looks like this:
http://www.spotcatch.net/component/users/?view=login
Maybe someone has an idea how to fix this...?