There is a problem with eliptic curves, see https://code.google.com/p/android/issues/detail?id=224438
It seems to be fixed in later versions of Android 7. If possible, try to upgrade your device or use a certificate without eliptic curves. (thanks Hervé and Marcel)
Auto sync issues
Some devices still seem to be affected by the doze mode bug, see Android 6
Apparently, the configured sync intervals get lost during the update to Android 7. What worked for us:
- Open account settings
- Change sync interval (whatever value)
- Close app
Now it should work again. Please don't forget to reset the sync interval to the prior value.
New accounts on Android 7 should not be affected.