After upgrading to 78.0b4, I had the following problem:

Compose a new message, enter myself as recipient, some random subject. Click "Send".

=> A dialog box appears with the text: "Unable to send the message, because there is a problem with your personal key. key ... isn't usable as a personal key"

I then check in the "Options" menu: "Do not encrypt" is selected, "Sign message" is not ticked.

From the main Thunderbird window, I then went to Extras > OpenPGP Key Management. I right-clicked on my key and chose "Key properties". "No, don't use this as my personal key." was selected. After changing it to "Yes, treat this key as a personal key.", sending the message worked fine.

Why would this option prevent sending a message that is supposed to be neither encrypted nor signed?

Please see
https://thunderbird.topicbox.com/groups/e2ee/T30c6a3b85c865020-Ma1357a5957ef5b327e1da0cf/for-testers-of-openpgp-configuration-change-required

and
bug 1648954

Thanks, so that's a duplicate.