It looks like part of the site, related to award travel booking search, was redesigned and has occasional glitches. Instead of completing the info on the page to which you are redirected, try the following.

After you log-in to your account, go to the homepage and complete the info for your itinerary. Before clicking to search, make sure to check off the box that says "Redeem Miles." This should take you to availability of flights and required mileage. This procedure works for me, without any problems.

Good luck; hope you can get your desired flights and dates.