Unable to change settings

Hi,
I’m trying out this plugin, but i ran into some problems. I have installed the plugin, but when i want to set my payment settings, I always got the message “Forbidden,
You don’t have permission to access this resource.”

I have checked the file premissions, those seem to be ok.
I was able to add an show, seating plan, princing, …
After trying above, I have deleted the plugin and reinstalled it (So i would have a “clean” plugin to find the problem with the settings.

Also, i don’t seem to find how to set the language settings to dutch.

Can anyone give some tips, or tell me what am i forgetting?

Thanks!`