MPOS Error: Creating device token failed

A little nugget of information:
In  Microsoft Dynamics AX 2012 R3 MPOS, if you receive an 'Creating device token failed' error when attempting to activate the POS or while doing transactions, you'll be halted from proceeding with what you are doing. The error will also say "- The value of Content-Type on server response is not supported."

A common culprit to this is that there is another terminal being used with that exact same Device profile.

The MPOS application can only be installed and activated on a per user basis, not for all users logging into a machine. So Sally can't log into the PC with her credentials and then the MPOS with her information while Tommy doing the same thing only under his PC AD credentials.

As a best practice, the users should all log into a generic account for that store/terminal where the MPOS is activated and then log into the MPOS under their own IDs.

UPDATE: You can have users log into their own respective accounts and have different MPOS Terminal instances. But they are not the same terminal.

Check the different users and see who is logged into what from a PC (Not POS) perspective as well as anyone in the backoffice or other stores.

Here is a scenario in which this could occur:
Someone may have accidently registered terminal 2 at store 52 as Terminal 2 at store 25, which would cause the store25 terminal 2 to get this error. Also, the person activating the device in the self-service model could've installed/activated the device under THEIR AD account, not the proper stores AD account.

Hopefully this helps!