An update to OC 8.2.1 solved it for one device, but not for another. An update to davdroid 0.9.1.2 solved it for the other device. So not too sure what the issue was exactly.
icloud Reminders Sync Problem
Reminders sync doesn’t work after Apple update the app. https://support.apple.com/HT210220
You can look the link. I can not write my problem clearly due to my English level. Sorry.
We know. When you update the Reminders in iOS 13, Apple removes CalDAV support for Reminders, so that they can’t be synchronized anymore.
I have added it to “Known problems” here, too: https://www.davx5.com/tested-with/icloud