A 500 Internal Server Error is like the check engine light on your car. It says something is wrong but not exactly what or where. You will need to go to your server's error log to find the error related to what is happening here.
Likely, this is related to an incompatible third party application/plugin. If so, you can try recovery mode here:
However, if it is an issue with the core software, we will require the error log entry to proceed.