I'm very sorry you are having this issue. I have contact our development team to see how to rectify this problem.
I appreciate your feedback on the Decal system and any screenshots and/or screen recordings will definitely help us refine our decal system faster!
I got the file back from support Monday (Friday today), and I made sure to save out all of my materials as files in case it would happen again. And sure enough it did - right after the first save. Re-opened the fixed file and deleted its decals and created new ones.
It has been working as expected up until now, as it just became corrupt once again; same error message. And I hadn't even touched the decals since I re-created them, and I've saved several times since then and been able to open it as expected. Until now.
This time I hadn't saved out the updated materials...
I reckon I'll have to wait another 1½ week to get this file back from support, as it took that long last time. And I told my client I would be doing the finishing touches today and hand it over to them at the beginning of next week...
This is a very serious problem! Really turning me off from Viz, I'm sad to say. Not even Photoview caused me this much grief. Well, that's not really true. But the bugs in that renderer were not nearly as irreversible...