This is usually caused when the Rest API service is down.
From the Avamar CLI, check the Rest API service:
admin@avamar:~/>: mcserver.sh --rest status
REST API Service is not running.
If the service is not running:
admin@avamar:~/>: mcserver.sh --rest start
Starting REST API Service.................................
REST API Service started.
Check that the service has indeed started:
admin@avamar:~/>: mcserver.sh --rest status
REST API Service is running.
You should now be able to log in to the Web UI.
Note: If the service is up, try
mcserver.sh --rest stop, then
mcserver.sh --rest start