It is definitely the grooves caused by the cassette rings. Twist each ring counter-clockwise to get them out of the groove before trying to pull off each ring.

Did you replace the batteries in the hub and computer before calling Saris? When the hub batteries are getting low, the round hub connection icon on the computer starts to blink. Even though the hub is fairly new, it may be a good idea to change the batteries and see if that makes a difference.