I’ve just checked the unicycle.com U.K. web site and from what I can see, the Ou-Ax luxus has spacers but the QX Series Marathon does not. I think you have the Marathon so there is probably no room for outside spacers. I think the problem is a bent crank. The Qu-AX light weight cranks are easily bent. Remove the two cranks and compare them to each other by placing them side by side on a level surface. The crank that’s causing the rubbing is probably bent inwards from a previous fall.
Also check that the bearing caps are properly secured. There should be no wobble or sideways movement of the wheel.
Hope this helps.