You need to log the actual error, probably a setting in PHP so you see what the 'internal error' is, not just that one exists. From there it'll make it a lot easier to find what's going wrong.

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator to inform them of the time this error occurred, and the actions you performed just before this error.

More information about this error may be available in the server error log.