Right, so the trigger for the bug appears to be upgrading from thunderbird 68.x to 78.y, as Joep did by enabling the ubuntu-mozilla-security PPA.
Note that that PPA is not recommended for general use, it is used for preparing and testing updates. This doesn't invalidate this problem though, which I haven't managed to reproduce in various testing scenarios yet.
Right, so the trigger for the bug appears to be upgrading from thunderbird 68.x to 78.y, as Joep did by enabling the ubuntu- mozilla- security PPA.
Note that that PPA is not recommended for general use, it is used for preparing and testing updates. This doesn't invalidate this problem though, which I haven't managed to reproduce in various testing scenarios yet.