Certain users are unable to authenticate into Android Inbox 2.12. They will continuously receive username/password prompts after launching the Inbox even if they enter the correct username/password information. This is due to the application incorrectly applying case sensitivity to this authentication prompt.
This issue has been resolved in the build of Android Inbox 2.12 that is available in the Google Play Store (as of Saturday, January 28th). If you have users who are experiencing this issue, please have them re-download the application.