Yes! Problem is solved for me, too.
Disabeling the CyanogenMod Privacy Guard was the solution.
Thank you all!
I have send already an eMail, but now I received the confirmation mail.
Equipment: Android 9.1 based Huawei P20, Calendar is acalendar+, Davx5, Thunderbird 60.x, Synology NAS DSM 6.2
Either Davx5 introduce a writeprotection or acalendar+ is doing so.
Can you please provide DAVx⁵ debug info (DAVx⁵ / Settings / Debug info)?
Do you use events with attendees/organizer?
It seems to be solved.
I was using the TBsync Addon in Thunderbird https://addons.thunderbird.net/de/thunderbird/addon/tbsync/?src=ss
Adding https://addons.thunderbird.net/de/thunderbird/addon/dav-4-tbsync/?src=userprofile to Thunderbird too solved the problems. Right now. So right now it was no problem of Davx5.
Thanks for help.
@Fraggle Thanks for the update!