Thank you @Dallas.
Yep, same issue, and unfortunately the workaround isn't helping in *your* account / folder.
For the record, the workaround is:
Instead of asking the server for the list of "all unread messages", for Yahoo the code asks for "all unread messages received since January 1, 1900". The date is arbitrary, should make no practical difference.
I'm sorry to see that it didn't work in your account folder... you'll have to wait for Yahoo's fix...