RESOURCE_EXHAUSTED: namespace count limit exceeded
Too many concurrent polls for workflow/activity tasks to a single frontend host. The default frontend.namespaceCount is 1200 and you probably need to increase it.
How much to increase, would check service_pending_requests gauge metric which tracks total number of outstanding polls and would tell you how many total concurrent polls server is seeing. (note this metric was added in server version v1.14.0)