It does not happen all the time, but when it does, you have to restart the ArcGIS Server before it is able to fetch the information from our web server. This is with 10.5 enterprise.
What can be done about this condition since our web server is up and functioning and the only way to solve it currently is to re-start the ArcGIS Server? It is happening frequently enough to be of concern.
Further explanation: we are rendering map images using the export map facility on ArcGIS Server map services by invoking the REST interface. Some of our layers use PNG symbols that reside on our web server and we pass the URLs to those symbols across to the ArcGIS Server. These symbols disappear when the ArcGIS Server stops communicating with our web server.