Sadly, Delicious seem to have broken their API when they recently updated their website. This affects all third-party clients including Yummy.
Without getting too technical, every API call tells Yummy that you have no bookmarks. This is why you don’t see any error message. This is also why there is no workaround that I can recommend you make nor any code changes that I can do to help.
Unfortunately this is entirely for Delicious to solve.
I will update if there are any developments.