They had to use another browser, one without and adblock to get in. Sorry but I'm not Yahoo support but here it's working on Firefox with the Ublock Origin adblocker.
I am logged into yahoo. I am on the main page and when I click mail I get a screen that says " here is your new yahoo mail address" it shows my yahoo id and email. It says click "get started" to continue. I press "get started" and it comes back to the same screen but without my yahoo id and email listed.

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