Minolta x-570: can't open the battery-chamber cover

As the manual instructs, I’m using a coin to unscrew the battery-chamber cover, counter clockwise. The problem is it just doesn’t move at all. I’m afraid that if I keep trying, I’ll mess up the screw (it seems to be made from rather soft metal) and I won’t be able to ever open it.

Is there any trick to get it open?