After I did a factory reset (twice; once through settings, once through recovery mode by clearing eMMC), I got a "low storage space" error. I can't get rid out of it - tried clearing cache and uninstalling every pre-installed app I could.
It's a generic Android device from Amazon (China) for $40, so of course, it reports tons of free space.
The main problem is that I can't install any apps. Google Play doesn't update. From the old store, I can't install anything, error 504. Installation of .apk's fails too, for an unknown reason.
Something weird happens that I can't understand: after a while, when I try to open Google Play Store, it says "app isn't installed". More curiously, restarting fixes the issue!
TL;DR I want to be able to at least install apps. The main issue is low storage. Factory reset doesn't work.