If you are a system administrator on the Tableau Server you shouldn't be getting errors when attempting to manage users. That error doesn't look nice, either. If it doesn't go away on its own, I'd try the following:
1. Restart Tableau Server. Does the issue persist?
2. Upgrade (if not on the latest build of Tableau Server - currently on 9.3.0). This may be an issue that has been fixed in a later build.
If you are on the latest build and the issue persists even after restarting Tableau Server, I'd recommend filing a support case. Include exact steps taken to reproduce the problem, screenshots of the error, and server logs as that helps expedite the case for the support team.