I've had three separate reports now for Conversartions where Thunderbird users have no default account in their Thunderbird, these two are the most recent, which seem to have been after upgrading to 68:

https://github.com/protz/thunderbird-conversations/issues/1392
https://github.com/protz/thunderbird-conversations/issues/1396

I'm not sure what's happening here, but Thunderbird shouldn't really be able to loose the default account, and if there isn't one, it should probably try and reset it, alert the user, or work without it.

Actually, this is the new state of things, it is no longer true that TB can't have a default account. It rather has none, than just any random one that doesn't actually fulfill the requirements for being the default. See bug 342632.

However, if there is another account available that can be default (as it looks from the github report), TB should pick it up. But that is bug 880602, which has been blocked so far. We should refresh it.
Also, the conversations addons should be updated to handle a null default account. The rest of TB already is aware of this.

Urgh, that's a shame that wasn't notified to add-on authors via https://developer.thunderbird.net/add-ons/tb68/changes

I've just raised this on the add-on developers list here: https://thunderbird.topicbox.com/groups/addons/Te61067ed1316b03d/nsimsgaccountmanager-defaultaccount-changed-in-tb-65

Though it could possibly do with being notified a bit wider.

So duplicate of bug 880602?

I think so.