If you are facing the Maximum Devices Logged in error in the DIGITS app, here is the good news. The bug has been identified by the backend software team. Debug is in progress and will be rolled out in an upcoming release.
Here is a way to circumvent the issue for the now:
If you have T-Mobile Visual Voicemail App installed on your device, please uninstall that and keep using the voicemail that is integrated into the DIGITS app. That Voicemail app and the DIGITS app is conflicting in some way to give that error.