Iv'e just been throught he same problem, and it seems you can't use a non-Yahoo email to login with, only your user name. Yahoo's 'Enter email address' on the login screen is somewhat misleading!
Doesn't recognise my login e-mail address (Yahoo screen) from within program but also doesn't work
either direct to Flicker.com.
Was with Yahoo for e-mail but now use BT e-mail (they originally used Yahoo or a tweeked version)
but then went their own way hence I wonder if this is the reason I can't log in.
I wen't through a 'transfer' process that may have closed my Yahoo account.
No hurry to re-register if that would sort it,with their recent past of major security issues
and massive loss of user data. Really don't trust Yahoo and if I can avoid them I will.
Sorry I can't help further.
You are welcome Fred.
In case you have a flickr account, any chance you give us you feedback as to whether you can login,
from within the application?
Tried 635 and all is well. Thanks for the prompt action.