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Are you using the 16.0.633 Octane plugin build with your Houdini version? It is a weird issue, I had not see these errors before, and here the materials system seem to work fine.
Can you please attach one scene with this problem to test it here?
These materials come from LiveDB, right? I think that the main problem is they have space characters in their names, something not supported by Houdini. You need to rename the material node, removing the space characters. Also be aware that the "Cracked_Paint" material is not valid, it is only a texture node, without a material node to apply it.