Android - how to keep launcher setting after reboot Instead of the preinstalled "Launcher3" I'd like to use "Microsoft Launcher" – but when I choose it as default, it only lasts until reboot. After reboot "Launcher3" is again set as default.

How to keep a launcher after reboot?
Where are such things stored in general?

(Android 5.0.2 - API 21)

EDIT:
My Phone is a Medion X5004.
I tried via the settings menu in MS-Launcher, and via system settings.

EDIT2:
I tested installing "Apex Launcher" and set it as default - also resets to Launcher 3. So it doesn't seem to be specific to MS-Launcher.

EDIT3:
MS-Launcher does not sit on SD-card.

EDIT4:
Wiping cache has no impact...

EDIT5:
Tried to chmod 440 /data/system/users/0/package-restrictions.xml while launcher set to MS-Launcher - after reboot the file has write permissions again and Launcher3 "greets" me.
Not really an answer – but problem does not occur after factory reset.