Happens to my Xiaomi phone after updated it to MIUI 8. Took me hours to figure it out!
Check the followings if you’re a victim too:
- Go to Settings -> Permissions -> Install via USB: Uncheck your App if it’s listed.
- Go to Settings -> Additional Settings -> Privacy: Check the Unknown Sources option.
- Finally go to Settings -> Additional Settings -> Developer options: Check the Install via USB option.
Update (MIUI 8):
Signin in to MI account is required to enable Install via USB option.
Update 2
For some devices (e.g. Redmi) you might need to turn off MIUI Optimization and reboot your phone.
Settings > Additional Settings > Developer Options > MIUI Optimization
P.S. don’t update MIUI unless necessary!
May the victims saved