If you've tried to log into your Flickr account from the Windows Phone Flickr app you're probably receiving an error message that reads,
"Unable to reach Flickr login services. Press Back to try again."
The error message is rather persistent, no matter how many times you press the Back Button and try again.
The problem? It's likely an API change that wasn't reflected in the app. The issue seems to be limited to logging into your Flickr account from apps such as the Flickr app and Pixl. In checking some of the apps that pull images from Flickr, but don't have the account log-in feature, such as Photostream and Flickr Wallpapers, they are still able to pull pictures from Flickr.