FYI, deleting the user's config under ~/.config, ~/.local/share and ~/.cache fixes the issue and geary starts successfully after that.
Of course that is a disruptive operation (user config is gone, and what about pop accounts that cannot resync emails later on). I can probably reproduce the issue on another computer if anyone wants to troubleshoot this issue further. -- Henry-Nicolas Tourneur mxid: @hntourne:matrix.nilux.be irc: hntourne on oftc