Just figured out that my calendars in thunderbird where not working anymore after the nextcloud 11.0.0.
A quick look and comparison I spotted the change, "caldav" changed to "dav" in the url.
Big thanks to the "Google to Owncloud, Contacts and Calendar" entry. This really helps me a lot.
Especially, the link above tells you how to configure your android phone and your mozilla thunderbird (with lightning) to setup the synchronisation with your owncloud.
I gave it a try with "caldav free sync beta" but had a long run of bad luck. After a while, I gave "CalDAV-Sync" a try and it is worth its money since it is working out of the box. Setting up the "carddav" synchronization is also a no brainer, but converting the google contacts to the owncloud contacts is something not that easy for me as a no-gui-user.
For "carddav" (address book) synchronization, you need to install sogo connector.
For "caldav" (calendar) synchronization, you need to install "lightning". The "howto" is described in the first link above.
I found one issue in my owncloud configuration file (config/config.php) and could solve it by adding "1 => 'my.owncloud.domain:443'" to the "trusted domain" array key.
So, I really gave it a try and invested multiple hours. Either my spirit is not strong enough today, or it is currently not possible to synchronize owncloud 8.0.2 calenders with other applications.
To calm me down, here is a list of possible problems that are the reasons I failed currently with my mission.