Add user gives error 500

[Strubbl]

i just wanted to try https://hallmonitor.info/user_watch/add, but it says

Internal Server Error

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

Please contact the server administrator at webmaster@localhost 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.
Apache/2.4.41 (Ubuntu) Server at hallmonitor.info Port 443