I have been getting a lot of these messages today when trying to access my notebook cluster (broad-dsde-firecloud/validation):
The server was not able to produce a timely response to your request.
Please try again in a short while!
Stopping and restarting the cluster is the only thing that seems to resolve the issue, but after a short time (~30 minutes to an ~hour) I lose access again. Refreshing my FireCloud browser and reopening the cluster (which seemed to help with similar issues in the past) doesn't do anything.