Hello,
Though I am not entirely sure what is causing this, it could be that the user is automatically logon with another user account he is already sigined in with. The easiest way to overcome this, is to create a separate browser profile and synch that to all other devices.
But best course of action is to first share the error message when the user logs on via the other device.
You mentioned that it does not work via an app on the mobile device. Does it work when using a browser session using desktop experience on the device?
Hope this gives you a start.