I have a very, very odd issue that's affecting all clients using our SharePoint 2013 solution from mobile devices.
If a user with read access visits the site from a desktop browser, everything works fine. If the same user visits from a mobile device they get access denied on every file from the themes folder, including SharePoint's own corev15.
If a user that has higher permission visits, it also works fine.
The error can be triggered from both an actual device, or by changing user agent in a desktop browser. But not all devices have this error, for example an iPad works fine, iPhone doesn't, and Kindle Fire doesn't, Samsung Galaxy S doesn't.
Only files in the
/_themes folder are affected. Files for example in
/_layouts can be gotten from every user agent and any permission level on the user.
How do I resolve this issue?