In the distance past, this error was often caused by FileMaker Server seeing itself on a second IP address ( sometime same NIC and sometimes second NIC )….
Perhaps that is still the case.
Save your work. Restart the Server and see if it goes away.
These errors often occurred when the server itself didn’t have a static IP address…. And so Filemaker Server gets unhappy when it sees another user using the same license….
It turns out its a FM bug, but one that's hard to find the solution for when googling. It turns out FM themselves know the problem and have a fix.
If anyone else runs into this problem, here is the link on how to fix it: