Everytime I download a new episode of "Back To The Future" from Telltale Games I end up with a screen that looks like the one below.
On the forum from telltale games it takes a while before a solution comes up.
I now found a solution (specifically for the mac) that seems to be the right way to solve it:
Try 'Command'+'Shift+'-' (yes the actual -), press yes, then 'Command'+'Shift+'+'. Then log back in.
An alternative option is to run the game as root. From a terminal run: