Redmine

Hello,
very weird issue.
I have setup redmine via passenger on apache with a very simple config:
/srv/www/htdocs/redmine is a symlink to /srv/www/htdocs/redmine_src/public and the symlink is owned by wwwrun:www (same as everything else for apache)

<Directory /srv/www/htdocs/redmine_src/public >

    Options FollowSymLinks
    AllowOverride All
    Order deny,allow
    Allow from all

</Directory>

RailsBaseURI /redmine
<Directory /srv/www/htdocs/redmine>
    Options -MultiViews
    Allow from all
</Directory>

When I visit server/redmine I see the login screen. Everything is fine.
When I try to log in. I get an Apache Server error 500 (not the redmine 500.html) which says that either server is overloaded or there is an error in a cgi script.

The apache access log looks like that:

xxx.xxx.184.187 - - [07/Mar/2012:14:23:46 +0100] "GET /redmine/login HTTP/1.1" 304 - "-" "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:10.0.2) Gecko/20100101 Firefox/10.0.2" 
xxx.xxx.184.187 - - [07/Mar/2012:14:25:06 +0100] "POST /redmine/login HTTP/1.1" 500 1122 "http://server/redmine/login" "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:10.0.2) Gecko/20100101 Firefox/10.0.2" 

The GET of the page worked fine but then when I try to login I get a 500. I don't know why POST fails.
The stupid part: There is nothing in the error_log about it. I have added LogLevel debug and restarted apache multiple times. Nothing changed.
Testwise I have set the entire directory /srv/www/htdocs/redmine_src/ to chmod 755, nothing happens.

The production.log says nothing about it. Looks like it doesn't get to redmine but apache does not indicate the slightest hint about what went wrong.

Thanks
Sam

P.S: Before I logged out I could browse redmine as long as it was GET, every POST like saving settings resulted in the same error 500