I ran into the same or similar problem running reports. Chrome would show this message: "This page isn't working. [myurlname] is currently unable to handle this request. HTTP ERROR 500". FireFox would show a blank page. Clicking on reload in either browser would create the report pdf. nuDebug showed no errors. Going to the my site logs I found a message that "allowed memory is exhausted."
Did a Google search '"[hostname] custom php.ini", found instructions for my hostname, increased the memory size, and this resolved the issue for me.