I believe this is the CalDAV known-issue, it's a NextCloud problem. It's been reported in this issue, there is a workaround:

Go to the config editor (Settings -> Advanced -> General -> Config Editor) Search for network.cookie.same-site.enabled Set it to false

For your own security you should set it back to true once this issue is resolved.

This was also mentioned in the release notes (very bottom of https://www.thunderbird.net/en-US/thunderbird/60.0/releasenotes/ )