This is a server error: the server doesn’t send ETags on GET responses, which are required for synchronization.
Please take logs and send them to your server admin/manufacturer. You may also post them here just to be sure.
After installing a new ROM into my phone and restoring the apps with Titanium Backup, I’ve been unable to sync contacts specifically; I was able to do so yesterday. The error is not appearing for the calendar or the tasks synchronization. The error seems to be caused due to an issue with the SQLite database being falsely marked as read-only. Neither reinstalling DAVDroid nor removing the data from the Contacts app solved the issue. Currently using Paranoid Android 7.1.2.
Is the file actually read-only (no write permissions)?