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
Since Android 6, there is the so called Doze Mode which puts background apps to sleep. Though in theory they should be allowed to sync, on many devices they aren't. For a fix see Android 6
With Android 7, Google goes even further and introduces the so called App Standby in addition to the Doze Mode. Now, apps are completely disabled when "the user does not touch the app for a certain period of time". Again, apps should be allowed to do background sync at least a few times a day, but that doesn't always work either.
On Samsung devices which all seem to be affected, the following steps might help:
1) Settings --> Device maintenance --> Battery button at bottom --> Scroll to end of list, and click "Unmonitored Apps"
2) Settings --> Device maintenance --> Battery button at bottom --> Battery Usage button under the graph at the top of the screen --> Top menu button to get to more options --> Optimize battery usage --> Switch to see "All apps" --> Find CalDav and Toggle the switch to off
See here for a pictured guide for vanilla Android.
We got many reports that despite following the above steps, the app didn't sync automatically. Unfortunately, there is no solution yet. We're working on this issue.
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.