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I've been using both those features for a while with my own Nextcloud server

Yes but you need tbsync to use it with nextcloud cal/carddav no?

I don't have tbsync and I don't think any other extension I use added support for that.

Simply select "On the Network" when adding a calendar, set a user name and enter the remote dav location. For address books use "Add CardDAV Address Book", also username and dav location.

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