Clicking Sign In does nothing. Clicking Play leads to the expected list of worlds, but most are locked and the ones that open are just white everywhere except mobs. Entering a friend's world turns their world also white.

I figured it was impossibly corrupted, so I uninstalled, cleared data&cache, and set up a totally new username. That worked fine for a couple of days, then it started happening again. Clicking Play went to the list of worlds under the old username.