Cheers Nick.
I think I may have found the problem...........
Since the lighting/lens setup I was using was identical to one I used recently (and which I had processed in 2.9.3), I imported a Scene Calibration preset from this earlier shoot by clicking on the preset tab in the 3.0 Scene Calibration. This opened the folder of old 2.9.3 Scene Calibration presets, and I chose the appropriate one.
Now, if I uncheck the Scene Calibration box, but leave all the other edits in place, the file save without a hitch.
So, using Scene Calibration presets from earlier versions of Phocus don't seem to work in 3.0.
Hopefully, this is the case.
It would be useful if someone could confirm it.
Thanks.
D.
Ps. If this is correct, where are the scene calibration presets stored in Windows so I can delete them? Thanks.