Hi Nick,
I have attached a screendump of the requested URL.
When accessing the AGS via the browser on the device, then I'm prompted for my credentials. As stated earlier, I'm not prompted when using the Explorer for ArcGIS.
If I logon to my organization on AGOL, and browse to the web map then I don't have any issues, and the map content is displayed on the map in the browser.
FYI - Matthew from support has created an incident and has been assigned to it.
Thanks,
Kasper