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Environment details?
A HTTP 500 error is an internal server error. This is a pretty vague error, and doesn't convey any meaningful information other than saying "Hey there is an internal server error".
You would need to consult your [unknown] webserver logs, PHP error logs, and MySQL error logs to see what is going on.
I doubt that re-installing osTicket would fix the issue, you would need to troubleshoot the server.