Ah you are using the new endpoint, not the old one. Then try the second part I added to NC12 Authentication issue with Cardav, Caldav and Sogo connector
Same problem here after update from freshly installed NC 11.0.3 to 12.0.
Thunderbird 52.1.1 (32-Bit)
Lightning 5.4.1.1
Sample URL: https://domain.tld/nc/remote.php/dav/calendars/username/calendarname/
Android client 1.4.3 over DAVdroid 1.2.2-ose has no problems.
Harald
Now it becomes quite strange?! 
I made the hack “second part”. (I missed, sorry)
Now, when I call the link in browser, I get the login.
But after login I get the same message “unauthorised”.
Unauthorized
Technical details
Remote-Adresse: xxx.xxx.xxx.xxx
Anfrage-ID: xxx-xxxxxxxxxxxxxxxxxxx
Unfortunately the connection in Thunderbird Lightning still does not work!
Same error …
My links if it helps
https://cloud.doamin.de/remote.php/dav/calendars/USER/cloudcalendar/
https://cloud.domain.de/remote.php/dav/addressbooks/users/USER/Contacts/
For Thunderbird and SoGo I always used the whole link.
For the Android carddav and caldav-sync apps I used just
the rest was automatically done by the app.
But even the whole link causes the authentication issue.
Thanks
Yeah, the browser is not supposed to work on those pages.
As for lightning maybe there is some cache in place, did you try rebooting the machine? And did you try whether creating a new calendar and adding that works?
The hack “second part” works… !!! 
Have created a new app-pin, removed the stored password in Thunderbird.
After restarting Thunderbird, he asked me for the new password. And everything is OK again.
Thank you again for your help !!! THX
I hope the problem will be resolved finaly by the next update 
P. S. Are there any restrictions on server security. Is there a loss of security due to the hack??
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I can confirm the hack works, but obviously this should be a temporary solution.
Thanks,
you might get a checksum error message in the adminpanel and can’t use the autoupdater any more.
Also think that should only be a temporary solution…
The only consequence should be that some clients can’t be used. But if it makes the ones work you actually use, that’s better 
Yes we will look for a real fix for 12.0.1
2 Likes
That’s right 
Perfect, thanks for the quick support!!!
Thanks a lot! We definitely appreciate your support!
Thanks a lot the second part works for me as well.
All my attachments are working again (Thunderbird (Ligntning & Sogo), caldav -sync & carddav-sync)
While I was checking some other devices that are attached to my cloud I noticed some strange behaviour.
I own two Sony Android devices and on both carddav and caldav were not working.
A google Nexus device had the same problems,
But on three Samsung Android devices there was at least carddav working caldav not. Could not figure out why.
Now with the patch everything is back to normal.
Hope for a solution for 12.0.1.
Thanks a lot for all the effort and good work.
Issue temporary solved:
Open issue => https://github.com/nextcloud/server/issues/5088
Just to add another person whose calendar is busted. A fix before the next release would be really appreciated! I tried this with Lightning, California, and Evolution on my Ubuntu machine. No joy.
I’m using Evolution on Ubuntu 16.04 and getting the
Data source ‘Ian Calendar’ does not support OAuth 2.0 authentication
error with my Calendars, but not my Contacts. I would have expected that both being webdav based they should be exhibiting the same error.
All seems to be well with my CalDAV-Sync and CardDAV-Sync apps on my Android phone.
I can confirm this issue with NC 12.0.0.
Hi Folks,
I think the WEB-interface have a similar problem with the authentication.
Please look into:
The fix in this thread has got CalDav-Sync on Android working again for me too. After the upgrade it was prompting for my password.