The CRC on this lens is done with the rear, in my experience with lenses with rear CRC is that its really quite durable and stable. The front element is the one that can be off, I've replaced the front element on a couple of 24mm f/2's and they all needed fine tuning to the torque of the retaining ring holding the element in order to get max sharpness.
If its still a right/left issue it could also be the mount, check to make sure all the mount screws are set and torqued properly. Your camera mount could also be out as well.
I just checked the back and all the screws seem be in tight. Nothing is rattling as far as I can tell. Fromt element and the lens tube seem to be on tight. When I shoot it with a Nikon D40, I don't see much issues at the edges like I have had on film.
However I did put on a very large lens hood and have also used a uv filter. Going to try without both and just a step up ring as a "hood". Seems like a long shot but will try it anyways.
Correction. On a D40 there is definite blurring only in the left side when wide open at F2. Stop down and it starts to go away. F2.8 it's still there. Again, left side only.
Yep, then it needs to be checked. The elements are set in an optical group together into the focussing barrel held in place by the front retaining ring which is where the filter and hoods are screwed into. If the lens has taken a knock to the front its possible that the barrel has come ever so slightly ajar enough to affect. If it were MY lens I'd be into right away. If you're handy with the screwdrivers I could talk you thru it.