Robert, here are the detailed logs:
2015-12-07 08:53:14 https://<myportalserver>/arcgis/rest/services/<myservice>/MapServer?f=json&dpi=96&transparent=true&format=png24&callback=dojo.io.script.jsonp_dojoIoScript1._jsonpCallback
2015-12-07 08:53:15 Matching credentials found in configuration file. OAuth 2.0 mode: True
2015-12-07 08:53:15 Service is secured by https://<myportalserver>/portal/sharing/rest/oauth2/: getting new token...
2015-12-07 08:53:15 Sending request!
2015-12-07 08:53:16 Token obtained: <mytoken1>
2015-12-07 08:53:16 Exchanging Portal token for Server-specific token for https://<myportalserver>/arcgis/rest/services/<myservice>/MapServer...
2015-12-07 08:53:16 Sending request!
2015-12-07 08:53:16 Token obtained: <mytoken2>
2015-12-07 08:53:16 Sending request!
2015-12-07 08:53:16 Renewing token and trying again.
2015-12-07 08:53:16 Matching credentials found in configuration file. OAuth 2.0 mode: True
2015-12-07 08:53:16 Service is secured by https://<myportalserver>/portal/sharing/rest/oauth2/: getting new token...
2015-12-07 08:53:16 Sending request!
2015-12-07 08:53:17 Token obtained: <mytoken3>
2015-12-07 08:53:17 Exchanging Portal token for Server-specific token for https://<myportalserver>/arcgis/rest/services/<myservice>/MapServer...
2015-12-07 08:53:17 Sending request!
2015-12-07 08:53:17 Token obtained: <mytoken4>
2015-12-07 08:53:17 Sending request!