Certain user can't join with the server. "Timed out", "Couldn't log in within given time", ...
This article explains the issue where specific player cannot join the server due to being "timed out" or "couldnt log in within given time".
If you have tried decreasing the View-Distance and Simulation-Distance and that didn't help, try to change the server location (node). Transfer the server to another location, but ask from client before if the IP change is OK.
