Just tested Profile Manager 3 on my own server and I have the same error with a brand new user.
In the Profile Manager Log, I have:
[93192] [2013/11/25 20:37:27.572] I: Processing AuthenticationController#device_callback (for XX.XX.XX.XX at 2013-11-25 20:37:27) [GET]
[93192] [2013/11/25 20:37:27.827] I: Redirected to https://mydomain.com/mydevices/
[93192] [2013/11/25 20:37:27.830] I: Completed in 257ms (DB: 55) | 302 Found [https://mydomain.com/authentication/device_callback?auth_token=[FILTERED]c927f-4355-401f-b45d-1d5113719401]
[93193] [2013/11/25 20:37:27.931] I: Processing DeviceController#start_ota (for XX.XX.XX.XX at 2013-11-25 20:37:27) [GET]
[93193] [2013/11/25 20:37:28.258] E: The logged in user is not in the devicemgr access group ({"succeeded"=>"true", "longName"=>"Test2", "uid"=> »XXXXXXXXXXXXX », "generated_uid"=> »XXXXXXXXXXXXXXXXX », "shortName"=>"test2", :authed_at=>1385429847, :auth_token=[FILTERED]> »XXXXXXXXXXXXX »})
[93193] [2013/11/25 20:37:28.258] I: Rendering 403
[93193] [2013/11/25 20:37:28.259] I: Filter chain halted as [:verify_user_access] rendered_or_redirected.
[93193] [2013/11/25 20:37:28.260] I: Completed in 329ms (View: 1, DB: 6) | 200 OK [https://mydomain.com/device/start_ota]
So it seems that all my users (old ones and new ones) are not allowed to access the My Devices pages.