When I try to use the RealCloth material there is a warning announcement, and if I ignore and continue the material is flat black, as youcan see in the attached images.
Just to make sure, have you updated to KeyShot 9.3 yet? You can download the Upgrade installer from https://www.keyshot.com/resources/downloads/
If the issue remains, it is because the objects in red do not have UV coordinates. You can correct this in ZBrush or through KeyShot. Although in KeyShot this may take a bit longer has you have to find each and every file showing this error.