It might be related to the number of connections Mailbird is using to connect to the server. We've had other users report it being fixed by limiting it, so try the following please:
Open the settings in Mailbird by clicking the top left menu button and then Settings, and then click on Accounts and then double-click your not working Yahoo account. Then set the Connections slider to 2 and click OK.
If it does not work, please contact support