For syncing Nextcloud (or other CalDAV) tasks with DAVdroid, you need OpenTasks, see here: https://www.davdroid.com/manual/introduction/#c215
Does that help?
I have DAVDroid running and configured to connect to a Baikal server. On the Baikal Server I enabled the iMip handler to send mails on my behalf. I created a new calendar entry with the Android calendar. From the server logs I can see that a new entry is being PUT. However, mails are not sent.
Does DAVDroid support iMip and iTip at all?
DAVdroid doesn’t support iMip and iTip (in fact, I didn’t even know that those exist until now).
I’ll close this because there’s no iMip and iTip support on DAVdroid. Maybe I’ll have a look at this later (and also see if it’s even in the scope of DAVdroid)