Oh boy, that's a new one... Is anything similar happening with the non-UI mesh? If not, then it's gotta be either Unity UI itself, or the code that sets the materials.
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Testing this out more in-depth now!
The additional shader channels variable seems to have been added in Unity 5.6, so this should fix it for that. But even though I'm following the sample code on the page here: https://docs.unity3d.com/ScriptReference/Canvas-additionalShaderChannels.html it still seems to cause a crash after a second or so. Are you applying the code in a different place? I'm applying it to the parent canvas immediately before the materials are set.
Setting the canvas to have the extra layers in the inspector also seems to crash. Not sure what I'm doing differently?
Taking care of this on your other thread: https://itch.io/t/190198/quadsimages-arent-displaying-at-all