I was on about a week ago, no issues. Sometimes the browser may be the issue. I was told to use Chrome for mission planning, thats the only thing I use Chrome for now! Clear your browser cache then close and re-open the browser. Try again. If you are using Chrome, try control-F5 whilst on the page... thats a hard refresh which will bypass any cookies that were set.
The main advantage I have found here is after you plan a mission there is a drop down menu that has a choice saying Export VLM...if you have Google Earth Pro on your desktop you can "playback" the mission and it be closer to what real flight will be.
In Firefox you can download the mission into Earth Pro but it isnt the same.