[TYPO3-seminars] 500 Internal Server Error when logged in

[Nebojsa Racanin]

Hello,

I've installed the Seminar extension under TYPO3 6.1.3. The server is running with PHP 5.4.20.
The extension is working well, if I am not logged in on frontend. But when I log in, I get only a "Internal Server Error", but just on the site, where the seminars should be listed. If i log out, the seminars are listed again, without any error.

Any idea where the problem is?

Greets
Nebi