I have the same issue on both my Nvidia Shield TV's. I log into the app okay, but after a day or so it logs me out automatically, after which I can't log back in. I get a generic "login error" message or if I try to use the Facebook login option it does nothing.
The only way to log back in is to clear the app data for the app, after which I can log back in again. I emailed Mediaworks support a few days agao but haven't heard back. Anyone else had this issue?