White Page And Networkerror: 500 Internal Server Error But Not Everytime

Hello All,

after login to my page www.skym.sk I get an error NetworkError: 500 Internal Server Error. Interesting is that this error is not show everytime and also when I tried

defined('YII_DEBUG') or define('YII_DEBUG',true);

there is no error.

Can it be problem with memory limits?

Thank you.. I am in production mode so thanks for every fast comment.