I have the same problem.
in one scene I have a rebuilt engine then a skeletal mesh. as soon as I assign an automotive material it goes into error and ruins all the instances that refer to that master material. who was a master opaque. to solve the error I had to remove the instances from the engine and physically go to the master material folder to replace the corrupt file. then I restarted Unreal and everything went back to normal. There is a solution? as they did in the demo since the machine was riggata and so I imagine a skeletal mesh?