Ah, thanks to firebug I saw a problem.
Graybox tries to load:
http://blog.diekellnerboerse.de/plugins ... frame.html
Do you have that file? Maybe you need to add a .htaccess with "RewriteEngine off" to that directory?
Wow, amazing work A .htaccess with RewriteEngine off solved the problem, but this should be added to the plugin for all users
I'll contact the author and post a link to this thread.
Thanks for your support. This one was really tricky, eh?
PS: Oh my god... (Autor: Thomas Nesges, Andy Hopkins, Lokesh Dhakar, Cody Lindley, Stephan Manske; version: 1.3) I hope i'll find one of them.