I found some information regarding this issue with Yahoo on their site. Apparently there are others that are or have been experiencing the same issue. Please check the link here for more information and possible solutions.
Re: Rogers Yahoo mail: 'your external email account is busy'
I do remember having something similar.. when i used to have my phone set up to access mail via PoP. When i had the mail app open on the phone.. it would do similar things on the WEB login, until i closed the mail app on the phone.