Thank you all for your extensive feedback! Highly appreciated!!
Just wanted to add that the described issue does not stop me from taking uw-photos 😉 , so nothing severe but annoying sometimes. Could be better, always trying to optimize...
My Nauticam clamps are of the same age, quite new (<100 dives) and regularly cleaned. So I think the issue is a combination of different slightly ball sizes between Nauticam & Retra (don't have the tools here to measure the difference accurately) and maybe user error regarding the Nauticam to Nauticam connections while assembling the setup before diving. By design the clamps can be slightly angled, maybe not paying enough attention to close the clamp really even is causing this.
As you can see on the pictures below, I swapped the Retra o-ring for a Nauticam one, but that didn't help much.
I have a Nauticam M10 strobe mounting ball for housing [25221], that does not match the Retra one and the thread looks quite different, too.