The app MUST be updated
To allow use of User certificate authorities (a CA certificate not from Verisign or such) the APP must allow in its security settings that user CAs can be used, and there are lots of settings around that.
The link that I posted in the initial OP provides an overview to what need to b done.
No DAVDroid to date have those settings, so for Android 7.0 only trusted certificates can be used, ie. letsencrypt.
Please, compile it in already. It can be tested in the AVD emulator.