New to GT this morning. Seeing same issue. Submitted helpdesk request first then thought I'd do a search on forum.
I am using
Apple MAC El Capitan
Firefox 41.0.2
Same on Safari.

Odd thing is it seem to be intermittent. I have just been through first stage of intro lessons. When I want to access 2nd stage I get the error. I watched the I Can't Quit You Babe intro, went to access is again and hit the error.

big not fixed?

thanks
nic