In short, new users need to log in using their Yahoo ID rather than a username and password that is unique to Delicious.com. Unfortunately they also changed the way that third-party clients, such as Yummy, need to authenticate users.
This means that if you have one of these new Yahoo! ID Delicious.com accounts, you will not be able to log in to your account with Yummy. I am working as fast as I can to provide an update that will work, but the new scheme is not adequately documented and there’s still the App Store review process gauntlet that needs to be tackled. This means that even if everything goes well it could be a couple of weeks before an update is downloadable.
If you’re an existing user I have better news: Yummy will continue to work. But if you are given the option to “upgrade” your account to use a Yahoo! ID, please say “no” until you have an updated version of Yummy.
My apologies if you are affected by this change but, unfortunately, it is entirely outside my control. Yahoo! provided no advanced notice which is frustrating to say the least. I would imagine that most other third party developers have also been affected.