A 500 error on a PHP request is pretty much guaranteed to be a crash in the PHP code. That could be caused by a code issue or a hosting issue, but regardless, the first step is to get the real error message, or you’re just stumbling in the dark.
The easiest way to get the error is to enable Display Errors and refresh the page. You can to that through the PHP Options menu, either in the client area or the control panel.
Could you try that and have a look and see if you can get the actual error message?