Need a Shader for the Label Material! Critical Error

[CopperSoffit]

I am working on some toy renders and all of the sudden I start getting a list of critical errors that all say, "Need a Shader for the Label Material!" and Keyshot immediately crashes.

This happens any time I open the Material tab, edit a material, or just switch to the Scene/Materials view (at the bottom of Project window).

A little background on the file: I have many objects that have a plastic material with multiple labels applied (1-3 labels per object). All worked as expected. I then had a couple objects that I needed to hand paint textures on, so I took those objects into ZBrush and used the Keyshot Bridge to bring them back into Keyshot with the textures. Again, all worked. To get the ZBrush texture applied to the already existing material with labels, I had to apply the material (while holding alt) to retain the Texture, then reapply the Labels. That worked fine. To avoid redoing the labels, on the second object, I used the Material Graph (new to me) and I wired the texture node directly into the existing material with the labels. Then applied the material to the object (so I had both textures and labels).
This also seemed to work, until I saved, closed and reopened the file. Now, I get the errors above and seem to not be able to gain any access to the materials without crashing Keyshot. The only "nuclear option" I can think of is to delete ALL objects with labels and import them one at a time, fixing/redoing the labels as I go.
I can't find any information on this error or why it started happening. The errors are numerous (about 15-20; I can't scroll before Keyshot crashes), so it is more than just the most recent object that I did differently. It seems that ALL labels have been corrupted somehow.

Any solution or insight into this error would be greatly appreciated.

[CopperSoffit]

Finding the offending object and replacing the material stopped the errors. Then I had to redo the labels. Not a BIG deal, but I'm still curious what causes the error so I can avoid it in the future.