@deekay No, it does not need any apps (except the system apps Calendar and Contact Storage, but those should be available on any Android system). Maybe there are some other "features" in this ROM, for instance a startup permission like here: https://www.davdroid.com/faq/synchronization-is-not-run-as-expected/?

Just tried it on my Samsung Galaxy S6 with Android 7 and I can confirm that the calendar entries disappear after using MAX POWER SAVING mode from Samsung. But only if I restart the device after the mode is activated. If I switch back from this mode without a reboot only the sync interval is set to manual (which is an already known bug from Samsung and can't be fixed by us). But at least the calendars are then still there. @Error101 you need to contact Samsung about it. It seems to be a common issue with their MAX power saving mode - people are affected in different ways by this

I'm using aCalendar+ 1.17.5 with DAVdroid 2.0.3 and new events are not tentative, but confirmed. Did you try another calendar app, like aCalendar+, Business Calendar Pro or Etar?

Now it is my time to confirm (it was quite long to receive the oreo update). Problem is definitely solved in Android 8.0.

Sounds like a problem of the branded Contacts app. Unfortunately, many Contacts apps assume that there can be only local and Google contacts If you really need this, you could try another Contacts app or a custom ROM like Linage.

Update - found the error - I had not seen the request for further permissions on the Note. Allowed that and all working fine now. Thanks

@crisch Hello, Unfortunately, we were never able to reproduce this problem, nor do I have an idea what the cause could be. I assume it's related to the Samsung firmware (as you can see, both devices are Samsung GT-I91*) which does something different than the default Android Contacts provider. Because Samsung's firmware isn't open source, there's no reliable way to confirm that or find a workaround. The only thing that I can suggest is to use an open-source firmware (AOSP, CyanogenMod etc.)