There error is:
HTTPSConnectionPool(host='myserver.mc.local', port=443): Max retries exceeded with url: /arcgis/rest/services?f=json (Caused by ProtocolError('Connection aborted.', ConnectionResetError(100
54, 'An existing connection was forcibly closed by the remote host', None, 10054, None)))
I have also discovered that this fails also when using the ArcGIS for Python API when trying to get a list of services in the same internal portal instance.