Failing to import emails from my yahoo account to Gmail. Help please

“ We were unable to connect to your old email account, probably because you are logged in to a different account with that provider in another browser tab. Please sign out of all non-Gmail email accounts in other browser tabs (for example, Yahoo Mail or Outlook.com), then close this window and try again”

I don;t understand - i have signed out of everything else, but it still won;t work. 

Can anyone help please?

Thank you