Time goes by, and this post didn’t get seen by anyone with time to answer it. This is a community forum, not a paid helpdesk. That said, let’s see if I can help.
We can’t share editing access to the same calendar right now
Nextcloud’s calendar app currently has a limitation - it can create and “own” a calendar, or it can subscribe to a read-only calendar, but it can’t do shared editing and permissions across instances right now.
You’ll probably want to watch this ticket for adding direct Calendar federation support.
But we should be able to subscribe to any calendar read-only across instances
I see at least one person mentioned sharing via link, but let’s make sure this was done properly. If someone tried using the CalDAV address, or using the public display web page for a share link, it would be expected not to work. Instead, make sure you’re using the “Subscribe” button from that page, as described here.
- You’ll need to hit the share icon on the calendar you want to share.
- Check the “Share Link” box, and click on the link/chain icon to open the public web display page for this calendar.
- You should see a “Subscribe” button in the top-right area of the page, near the “Download” button. Right-click on it, copy the address that it goes to, and you should now have a URL similar to this:
- On your other Nextcloud instance’s Calendar app, hit “+New Subscription”, and paste this URL into the “iCal Link” field.
Since my Nextcloud is behind a reverse proxy setup and I don’t have another instance to test against, I can only verify:
- The URL works correctly - passes the iCal validators, and works as expected with Thunderbird’s Lightning plugin
- Other valid
webcal:// links work just fine as read-only subscriptions in the Nextcloud Calendar app.
If this still doesn’t work, it’s worth filing a bug over, but please do make it clear that you used the iCal export/subscribe option rather than attempting to reach it via CalDAV.