Yes, this can be a real bugger to shift for the unwary. Its a pity that you werent forewarned by the retailer you bought the machine from. Pretty well all specialist retailers will advise you to loosen this screw before first using the machine and to retighten just beyond finger-tight.
Since you werent advised to do this, the constant heating/cooling cycles have woven their magic and all but "frozen" the screw into place. About the only thing I could suggest, is to apply some Freon (Shhh) freezing spray directly on to the screw and then very quickly try loosening it before the whole metal mass balances out the thermal imbalance. You will have to make sure you are using a driver with a new bit installed or a new screwdriver with nice sharp edges on the tip.
Failing this, you might have to locate a machine shop or some other workshop environment where they may have small right-angle drive impact wrenches in use. I dont know that a standard small impact wrench would be able to fit in the gap between the Group and the Drip-tray shelf. Hopefully the freezing spray will do the trick though. All the best and good luck,