I bought the DAVx5 long time ago (for Android) and it always worked with my Nextcloud on Pi.
I was very happy w/ this nice little tool.
But since some few weeks I cannot sync any more. My pad w/ WiFi only can not at all and my phone (Nokia 8 w/ Android 9) only when I disable WiFi connection (-> over mobile connection 4G).
I deleted account in DAVx and configured new, but it cannot find services. Also I playes w/ WiFi only sync option in DAVx but it does not help.
PS: Outlook w/ CalDav plugin works as well.