I've added some arguments to our script that launches sync_gateway. It now looks like this...
nohup /opt/couchbase-sync-gateway/bin/sync_gateway -verbose=true -log="SYNCVERBOSE,SYNC" gateway.conf.json > /tmp/sync.log &
...and we get HTTP and HTTP+ messages in the log, but nothing that indicates a sync error. I tried with camel case Sync, and SyncVerbose, but didn't see a difference in the logs. The only thing that did make a differense was the -verbose=true flag, which added the HTTP+ log messages with response times.
We are also monitoring our pull and push replicators on the client, and don't see a whole lot of meaningful messages there either, but I'm going to make another attempt at reproducing and watch the logs on both sides. Any other potential suggestions?