Inner wall printing speed is by default twice the outer wall printing speed. For some reason (defective noozle, defective extruder system, installation of a stepper damper on the extruder ... ) higher speed could lead to underextrusion. Therefore, inner wall gets printed too thin. That make gaps between walls. If you reduce inner wall speed (make it equal to outer wall speed), you might temporarily fix these gaps. But the best way to solve this, is to find out why extrusion is too low at higher speed.