Trying to replicate but can't paste screen shots well.

After I choose my flights, I get this pop-up:

Insufficient Miles

We’re sorry, your balance of x,xxx Miles is not enough to redeem this Award Ticket. Start a new search or return to your search results to make a different selection.

With choices below of "Start New Search" or "Return to Results" (or x-ing out of the pop-up in the upper right corner).

X-ing out brings me back to book a flight.
Return to Results brings me back to choosing an outbound flight
Start New Search does what it says.

Thanks for the suggestion to try this when not logged in. Counter-intuitive, but that solves the problem..... (Odd).