Error accessing payments on back office

[VikG]

I get this error when trying to access the payment link on Back Office. Does anyone know why?

Server error
The website encountered an error while retrieving admin1/index.php?tab=AdminPayment&token=3cae6597c97cf5d0b036d5a2b398b2af. It may be down for maintenance or configured incorrectly.
Here are some suggestions:
Reload this web page later.
HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfil the request.

I was trying to work out why when the order goes to paypal it comes to 20% less than the basket total (i.e. the total VAT amount less than the order total). Is there a reason why? Is this a related problem?