One week after complete reinstallation of my OnePlus I started getting one “Unfortunately, Google Play Services has stopped” error message after the other. I tried a lot of solutions proposed online (“clear the app cache”/Goto Privacy guard/…) but none worked. Until I found this one from ‘ianyeo’:

Seems like lots of users getting this annoying pop up. Here is the fix. No data will be lost.
1. Go to “Settings”
2. Select “Apps”
3. Swipe left until you in the ‘All’ column
4. Click on the 3 dots in top right
5. Select “Reset app preferences”
6. Reboot OnePlus
7. Reselect preferred launcher
8. Thank me
9. Done! Enjoy!

via https://forums.oneplus.net/