The Android app authenticates user credentials for the first time by making a GET request for user-tags on the VA artist:

http://www.musicbrainz.org/ws/2/artist/89ad4ac3-39f7-470e-963a-56509c546377?inc=user-tags

The realm is set as: musicbrainz.org
The scope is set as: www.musicbrainz.org

This worked fine and users were able to log in, but it stopped working a week or two ago. The request now returns a 401 status code even with valid user credentials, which have been tested on the website. Nothing with regard to authentication has changed on the app side of things since it was released.