Actually, you were right. The issue for me is that the original k9-mail repo did also released the current beta thunderbird version (and i think, one with still the k9 branding if needed). I was trying to switch to it via that repo, and didn't noticed the thunderbird fork listed the in the repo. There's no changes, outside of the releases only being thunderbird branded outside of all the previous k9-mail ones. If you were on k-9 mail, and want to switch to it on Obtainium, manually install the new version and mugrate your settings to it. And once it's all done, remove the k9-mail repo from Obtainium, and add that thunderbird one.