White page after submitting order

[pingelmonster]

Dear users,

 

I am implementing WHMCS with my organisation. When I register myself as a customer and place an order, I get a blank white screen.

 

I do receive an orderconfirmation with PDF-file, but I cannot return to the homepage without customizing the URL.

 

I've added a screenshot, see the attachment:

 

Do you know where this error occurs and how I can fix it?

 

Thanks in advance