The login-config.xml is invalid. The last exception comes from trying to parse the file as an auth.conf format file after the xml parse failed. Look in the server.log for the original xml parse error.
Unfortunately, the server.log gives exactly the same error message.
Something even stranger: when I comment out my new policy, the error still occurs, although the uncommented XML is the same than the original file.
Some other test I did: duplicating an existing policy (and changing its name of course) leads to the same exception.
I found the problem: there was an "é" (French character) in a comment that I added. So now when removing the comment the parsing succeeds.
This is really a tricky error. I am not sure whether it is a JBoss bug or not.
From now on I promise to use only English in comments ;)