A weird suggestion, but if you created the exception using the "Add another requirement" link near the bottom of the Availabilty Settings pulldown, go there again to check that some odd Start Day requirement hasn't appeared. I accidentally noticed just that tonight. Every exception I'd set suddenly had a "Must start on a Sunday" requqirement, even when, for example, there wasn't even a Sunday in the date range, or Sunday was the final day of the range. I had NOT set that requirement, and it had NOT appeared there before. That kind of bug could've kept anyone from booking during those periods if I hadn't noticed it. Clearly yet another bug in AirBnB's code, I'm sorry to say.
This is the 3rd money-costing bug I've run into in just the last two days. Kinda disappointing.