arcgis server - Secure Print Service Failed to Execute

I have a web application that calls an ArcGIS 10.2.2 secure print service (accesses secured map services). I am calling the print service using a proxy page. Generally this works, but after roughly 30 minutes the print request fails for no apparent reason, and the ArcGIS Server logs contain the following:-

    Error executing tool.: Layer "internal": Unable to connect to map server at 
    http://myserver:6080/arcgis/rest/services/Internal/Cadastral/MapServer. 
    Layer "jobs": Unable to connect to map server at 
    http://myserver:6080/arcgis/rest/services/Internal/General/MapServer. 
    Layer "wells": Unable to connect to map server at 
    http://myserver:6080/arcgis/rest/services/Internal/Oil_and_Gas/MapServer. 
    Failed to execute (ExportWebMap). Failed to execute (Export Web Map).

The error reported back from the http request is:

Error: Unable to complete operation

The rest of the application works perfectly. If I try printing later (say 20mins), or if I restart the Print Service Tool, it happily prints again for another ~30 mins. It is as if a connection is timing out. Token expiring? Has anyone had this problem before, or can someone suggest where I should direct my attention?