This appears to be a known upstream bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1727062.
** Bug watch added: Mozilla Bugzilla #1727062 https://bugzilla.mozilla.org/show_bug.cgi?id=1727062 ** Also affects: thunderbird via https://bugzilla.mozilla.org/show_bug.cgi?id=1727062 Importance: Unknown Status: Unknown -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1945368 Title: Thunderbird 91 forgets default calendar To manage notifications about this bug go to: https://bugs.launchpad.net/thunderbird/+bug/1945368/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
