Same issue,
I can visit back office but can't visit modules page.
Click on menu item Modules> Modules is causing "HTTP-fout 500 (Internal Server Error):
Same for menu item Modules>payment "HTTP-fout 500 (Internal Server Error):
Difference is that I am having this problem on clean install, no upgrade.
Upgrading from 1.8.4.2 is causing the HTTP 500 error in FO and BO.
Suggestions to solve this are welcome.