Does anyone know how to resolve this?
I think it’s a sync issue between the client and the server. Try to log to ymess with web interface or from the e-mail interface itself and add the buddy there.
That didn’t seem to help.
Hmm, this used to fix it for me when it did happen. I did a search on pidgin trac and I saw that sometimes is related to adding the new buddy by using @yahoo.com and not using only the YM id. Is this what you did?
If so try removing that @yahoo.com buddy and add him / her in pidgin only with the YM ID.