Try setting a breakpoint at GTMOAuthSignIn's requestRedirectedToRequest: method to inspect the responses.
I get an error:
MyFlickr[7637:11603] didFailLoadWithError: Error Domain=NSURLErrorDomain Code=-1004 "Could not connect to the server." ......
It really seems that something has changed in Yahoo oAuth services, now I'm looking for a place where to aask for further info