Is this happening for gmail accounts with 2SV? If so, do the accounts use OAuth2 authentication or an app password? See the 'Two step verification' section here.

gmail and yahoo both use OAuth2 authentication. The link to 2 step verification section above does not seem to work. I did not set up 2 step verication on either account; but it is activated with the new 68.1 version of Thunderbird. How can I turn that off?

TB currently has no support for 2SV or 2FA. But if the account has it set up, OAuth2 authentication allows the account to be added to TB.

Here is the section from the link that is currently down; this is from the cached google page.

"The Google Account help advocates using two step authentication. That sends a code (a minimum of once a month but ideally every time you log in) to your cell phone that you need to enter when logging in using a browser. However applications such as email clients can't do that. If you configure two step verification you need to either use OAuth2 to authenticate or create an application specific password for Thunderbird that you use instead of the normal password. The application specific password doesn't change when a new code is sent to your cell phone. If you created a application specific password, you use it for a while, and later on its rejected, forcing you to create another one, try deleting all google cookies using Tools -> Options -> Privacy -> Show Cookies, exit and restart Thunderbird. [18]

Two step verification is not needed, and not recommended. Use a strong password, and don't use the same password with other email providers/web sites, instead. However, if you have another email account it is a good idea to set a recovery email address in case you ever forget/lose your password."