I had this problem yesterday evening, and was hoping it would be a temporary thing. Unfortunately, I'm having the same trouble today. What I'm trying to do is book MFU tickets to Spain in April. I see flights with availability, e.g. Only 5 reward flight seat(s) left in this class. When I select the flights and click 'Continue' I get the following message:

Error
Sorry, there's a problem with our systems. Please try again, and if it still doesn't work, you might want to try again later.

I tried different dates, but no dice. I don't know if it matters, but the flights are on Iberia. Is anyone else having similar problems today?