Same issue here, called the agent and they found plenty of options with Ethiopian. I could book over the phone but not online. They did NOT wave the 20 € fee for booking over the phone which I thought was just rude (since it's their online system that doesn't work).

Look for the award flight using another miles program (I used Lufthansa) and all Ethiopian flights there could have been booked by the agent over the phone when I tried.

Good luck!