Synchronisation between Thunderbird and Nextcloud CardDAV and CalDAV

Thunderbird does not work with calendar or contact data from other servers than the mailserver.

Problem

Newer Thunderbird blocks connections to foreign domains by default, presumable for scurity reasons. If the mailserver and the carddav or caldav server have different domain names, thunderbird blocks the connection.

Solution

See here https://github.com/nextcloud/server/issues/10134#issuecomment-411322692