thank you a lot @ all who have tried ans especially
Dave R and kaas!
Upon another close inspection I found out that one edge consisted of only 10 faces while all 3 other "rounded-edges" had 12 rounded faces...strange.
Plus I could clearly see those "precision related errors" Dave was speaking about,
I must clearly have messed something up big time unintentionally by being a very few mm off without noticing it!
The red-axis for example does not run parallel to the objects outer edge.
maybe it is so slightly off, that it is "ok" for the object as it is, but once solid-actions are performed it just looses it.
Anyway. Am happy to have a working clamp now and will pay even more attention to those little details in the future.
thanks again to everybody who wrote in this thread.
best regards,
Alex