Yep. That's it. I found a post here that led me to a post on pnet with a fix for the 35 Ultron. I just did a couple of searches here and couldn't find what I was looking for.
IIRC, it involved a bit more than just taking off the front ring and tightening that first set of screws. The repair inolved taking out the first set of screws, removing the aperture ring (being careful not to lose the ball bearing or spring for the detent), taking out another set of screws and then finally to the screws that cause the problem.
Apparently this final set of screws would loosen up a bit causing the play. The fix is to back out those final set of screws, put a bit of nail polish on the threads and re-install, hence fixing the problem.
I remember the guy that posted the fix saying that he contacted Gandy about it. Gandy's reply was that it was a common problem - $150 to fix! This fella did it himself in about 15 min.
Try some searches here & on pnet. I dont know if this fix would work for the Nokton, but it's so similar in constuction to the Ultron....
I wish I could be more help but I can't find the thread I'm looking for!