UMP will stop working, and there will be a popup error. Refreshing, restarting does not help. Checking wasp log and TCP port usage for port 80 (the default) it can be seen that wasp can no longer connect to port 80.
To temporarily get it working, reboot the server hosting the UMP robot.
This issue is resolved in KB4338831.
This can also happen if the "World Wide Web Publishing Service" is running on the UMP robot.
A workaround is to reconfigure wasp to use a different port like 8080.