Just like how you can have an ad disapproved ( Inaccurate Display URL, Inappropriate Content, Spacing and so on), we have now just had a keyword disapproved, for "Display URL".
The merchant we are are promoting is, according to QuantCast "is a top 5,000 site that reaches over 359K U.S. monthly people."
When I checked our link, the site was down. One hour later, they are back up. Presumably it was just a temporary problem that is now fixed.
I'm guessing the answer is to ask for it to be un-disapproved...