Plugging that into "Errands" didn't work, it resulted in a 404. Indeed, the above URL returns a 404 when contacted directly. Fastmail makes use of the CalDAV auto-discovery (outlined in https://datatracker.ietf.org/doc/html/rfc6764) which essentially says:
Same situation with Baikal, but baikal requires https://caldav.{server}/dav.php/calendars/ because server differences. Would be more convenient to be able to use the base domain directly
I am using fastmail which uses https://caldav.fastmail.com as their entrypoint for CalDAV. See https://www.fastmail.help/hc/en-us/articles/1500000278342-Server-names-and-ports#h_01H95ZZXYQJCB2E8CEM5PDK8H0 for details.
Plugging that into "Errands" didn't work, it resulted in a 404. Indeed, the above URL returns a 404 when contacted directly. Fastmail makes use of the CalDAV auto-discovery (outlined in https://datatracker.ietf.org/doc/html/rfc6764) which essentially says:
<host>/.well-known/caldav
Indeed, Errands correctly imports all tasks when the Server URL is set to
https://caldav.fastmail.com/dav/calendars