Authentication not working, Shield locked me out (on fcgi)

Hey there,

I'm having a hard time getting Shield running on PHP 5.5 / FastCGI*, although I added the required rewrite rule.

After setting a user and password the authentication dialogue pops up but won't accept my credentials (I can rule out typos, tried and reset them several times).
If I switch to running not as FastCGI it works. But I want and need PHP to be running under FCGI, so are there any further things I could try?

Cheers,
Daniel

*PHP-FPM (FastCGI Process Manager)
I just found out my host doesn't use classic fcgi but FPM, got no idea what means: http://php-fpm.org/