Thanks Luis,
Perhaps so. But I have heard of many others experiencing this problem beyond this particular thread.
I have deleted the prefs, with the technique you describe, and the problem persists.
I am on the latest verision of Motion (5.2.1) and on Yosemite (10.10.2) am on a new (cylinder) Mac Pro (Late 2013).
Graphics card: AMD FirePro D700.
I do have several plug-ins running with Motion (Reel Smart Motion Blur, Twixtor, Red Giant Looks and few others) but figuring out which one may be conflicting with this seemingly simple task of saving a current frame, is a bit of a needle in a hay stack. I actually get an error message from Motion when trying the save current frame.
Or if it's a Permissions problem, I don't know how to solve that... why are Permissions in their default state, often a culprit with Mac problems?!