Your editing the wrong login-config.xml I would guess.
This was my first idea, too! I edited the server/all/conf/login-config.xml.
I checked whether I got the wrong one:
I've added a filter creating a LoginContext with my newly added security-domains' name and it works.
I really have no idea what's wrong.
It was a really thumb error!
Wasn't the config, but the JSP!