Unable to log in - "Only one account can be signed in at a time."?

by pbyrne
Question by Peter E: Unable to log in - "Only one account can be signed in at a time."?
I've been a Yahoo! user for over a decade and have never had this problem before.

I tried to log in yesterday, and couldn't - there was a message that "Only one account can be signed in at a time."

I left it, and tried again today, getting the same message, and still unable to log in.

I am not, as far as I'm aware, logged in on any other computers - certainly not in this building (and I

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