Calendar sync issue

As of Thunderbird version 60 many people experience calandars stopped synchronizing with Nextcloud.

Workaround

1. Navigate to: Preferences > Advanced > Config Editor...
2. Press: Accept the risk!
3. Search for: etwork.cookie.same-site.enabled boolean
4. Doubleclick the etwork.cookie.same-site.enabled line to set it from true —> false

Also see: Nextcloud forum