No chance to create a category

Hi,
yesterday I installed the eventon component and all plugins and modules of it. Then I tried to add an category. But it doesen´t works. All I can see is only a white screen. Is there somebody with the same problem or/and a solution?

Hi Andre Amgerg,
Thanks for contacting us and choosing our product!
Our product category. I used the Joomla category. You can check back.
If you have problems. Send me your account site via private message, please. I will check help you
Thanks!

Everytime I try to click on Category I get

Fatal error: Call to a member function xpath() on a non-object in /home/histor08/public_html/libraries/allediaframework/Framework/Joomla/Extension/Generic.php on line 457

Doesnt do me much good if this doesn't work.