I'm on another try to use cloud calendar synchronization, across my desktop, laptop, and at least one (to start) handheld mobile device.
The cloud side is a community-run Nextcloud instance.
On the laptop and desktop, I'm trying to use Thunderbird, but it was silently failing to sync.
I had to rummage around to find an error console,
hamburger menu -> Tools -> Developer Tools -> Error console (C S j)
at which point, I see
Lightning:[calCachedCalendar] replay action failed: null [...]
A little searching finds several things, including something about corrupted SQLite entries and the need to clear them, but what worked for me is
https://github.com/jobisoft/DAV-4-TbSync/issues/35#issuecomment-439308002
which points to a now already-bitrotted (after less than a year) link, not available through The Wayback Machine.
Fortunately, a workaround is noted in the issue, toggling to false the state of this setting:
network.cookie.same-site.enabled = false
which can be excavated by
hamburger menu -> Preferences -> Advanced tab -> Config Editor ...
I suppose I should see if either Nextcloud or Thunderbird have bugs for this, possibly even something that's been fixed in a release newer than what I'm working with.