I get this on occasion, though I do not use Yahoo.
I have come to the conclusion that my system thinks I am already signed into my account. If I don't sign out and quit my mail program, then I often have to wait to get back into my account because, I think the system believes the account is still open to me already. I have to wait for about 20 minutes, then, to gain access.
I don't know if this helps. But using several machines to access the one account means there is an increasing likelihood that the system is already in use (by me) if I try to sign on again.
That's my explanation, and my cure is to quit mail and wait. I can always use webmail if it's vital.
Hope this helps.