Hello,
I was getting the same error message until I have checked the Contact birthdays
calendar to sync in CalDAV as it was intentionally not set to sync from the very beginning.
Right now, I am not getting that error message again, even despite that I have force closed DAVDroid and reopened it.
Now I cannot even see my calendars anymore 
I am attaching two debug logs and a screenshot.
According to the debug log, the only difference is the sync row 0 vs 1.

Update
Hum, that is interesting… A basic reinstallation of DAVdroid 1.11.3-gplay
was not helping, but when I have installed 1.9.9-gplay
, the sync worked as expected. Then I’ve installed 1.11.2-gplay
and it worked too. Then I have installed 1.11.3-gplay
back, and so it has also worked.
I am confused now
The good point that it is working now for me. (without me cleaning the brute force attempts table, see below)
If you anyone is having the same issue while using Nextcloud or Owncloud, please try cleaning your brute force attempts table from it:
1. stop your nextcloud/owncloud instance;
2. backup your nextcloud/owncloud database;
3. delete records from the brute force attempts table:
$ sqlite3 /path/to/nextcloud/data/owncloud.db
sqlite> BEGIN;
sqlite> SELECT * FROM oc_bruteforce_attempts;
sqlite> DELETE FROM oc_bruteforce_attempts;
sqlite> SELECT * FROM oc_bruteforce_attempts;
sqlite> COMMIT;
sqlite> .quit
4. start your Nextcloud/Owncloud back again and try DAVdroid to sync the Calendar again.
Kind regards,
Andrey Arapov