https://www.getmailspring.com/setup/access-q-com-via-imap-smtp

I would be guessing. Yahoo has indeed changed with it's move to Oauth and changes by its new owner.
You can try looking at Yahoo.com and your email via the web page to see if it complains as well then fix that.

As to Q, it's they app/system so while I may try removing and adding the Yahoo.com account in Q's world, if that doesn't fix it, they should be inundated with support calls like yours and have a ready answer.