Just wanted to post this, before someone else also tears their hair off...

Baffled and confused I just realized that transparent renders where no longer completely transparent!? But instead had a "greyish" overlay. I rendered out test image after test image, changing settings, starting empty new projects, with or without cameras, changing all the settings in the Visual settings box. But could not get rid of the nasty grey. Searched these forums for transparency threads, to no avail.

Finally I discovered a little setting in the seldom-used Post Effects window. With iClone 7.3 came a new standard post effect called Static Noise.

So, just in case you are as baffled as me. Untick that ;-)

It's a good idea, but shouldn't be preticked in my humble opinion. You can read about the purpose here:
https://forum.reallusion.com/380904/06-iClone-73-NEW-RELEASE-Visual-Improvement?Keywords=static%20noise

Here's the effect on and off on a transparent layer placed on a white background