error 500 internal server error

Hi I have recently installed this and occasionally I cannot download the PDF and just get an error page 500.

It seems to be only on orders with larger numbers of product lines. I have increased the memory in the Config file and the ini file and checked the status and all is green except the logo which is orange and would prefer a png file but I don’t think this can be causing the problem, can it?

I don’t know what to do next, happy to pay for the paid version if it’s sure to work.

The page I need help with: [log in to see the link]