The log provided shows each _resync request returning with a 503 status, does the log align with the failure scenario you describe?
Can you run the following curl command and paste the results here:
curl -X GET http://host:4985/_expvar
If you look at the stat “syncGateway_rest”, if there are no other active users on Sync Gateway, “requests_active” should be ‘1’ (your _expvar request). If there are other blocked requests then this number will be greater.
e.g. for an SG instance with no active users:
"syncGateway_rest": {"requests_0000ms": 3, "requests_active": 1, "requests_total": 2}