I do not really have problem. But I found a modified behavior of DAVDroid somewhere in the past, probably in the course of 2016, which makes syncing complicated now.
I use a Samsung phone, original firmware, Android 6x, rooted. Rooting was necessary in order to install a firewall. Until somewhere in 2016, it was sufficient to open the firewall for DAVDroid (I can choose for which processes/apps to open the firewall). It is not so anymore. Opening the firewall for DAVDroid only does not allow for synchronization (no error messages from DAVDroid). I did not have the time yet to puzzle out for which other process/app I have to open the firewall additionally. Therefore, I deactivate the complete firewall for a short time, when I want to synchronize. This is poor.
Which (system?) process is used by DAVDroid additionally to accomplish synchronization?
Why did it work before?