'Connection timed out' issues on Portal 10.5 typical culprits?

[CassandraFollett]

Users are having issues publishing map services to Portal (Federated with ArcGIS Server). In the logs, errors say "[URL mapservice] is not accessible: Error. Connection timed out"

(For example,

"URL 'https://[domain]/Test/MapServer?f=json&token=[token]' is not accessible: Error. Connection timed out (Connection timed out)."

Meanwhile, on the ArcGIS Server, I see the following error,

"Unable to get service usage timeout for [mapservice]. Defaulting to server directory timeout. GP Service [mapservice] not found"

Not sure if it's related, but it's filling my logs.

Obviously on the client side users are having their requests to publish take so long that the connection is dropping, but is that typically a server performance issue or a networking/firewall issue? Or if it's unclear from what I've presented, what's the best way to proceed with diagnostics for this?

(Environment:ArcGIS Enterprise (Linux)  Portal with federated ArcGIS Server, using Apache)

[CassandraFollett · Accepted solution]

To follow up on my own question,

It seemed to be an issue of pooling. I increased the maximum amount of instances for the Publishing service, and this seemed to alleviate the issue. So, if multiple people are wanting to publish simultaneously, I would recommend increasing the maximum instances for the Publish service.

The other issues mentioned in my post still exist.