I have now reproduced the problem on Android 7.0 in the SDK emulator image:
a sample ECC certificate (secp384r1) does not work on Android 7.0, but
this ECC certificate (secp384r1) works with Android 8.0
However, I couldn’t see any differences between DAVdroid versions. I have tested DAVdroid 22.214.171.124 and DAVdroid 1.2.2 with Android 7.0, and it does’t work, too (as I have suspected).
So, in my tests, this is the Android 7.0 bug which has been fixed in Android 7.1 and is not related to DAVdroid.
I don’t have an explanation why different DAVdroid versions have behaved differently in your environment / testing setup.