"it would have been a relief if that was the problem"

Just so you know... in my opinion the above is proper contemporary American idiomatic speech.

"I would have been relieved"

Is more formal in American English as I know it... that usage would seem old fashioned and in film or book dialog might signify the speaker was using UK English.

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I don't know what glitch might be causing this... have you used a text editor such as EditPad Lite to check the neck section of the .cr2. To me it seems maybe an invisible character or such has corrupted the body part name.

Have you tried re-rigging the geometry with a fresh M4 .cr2 in Setup Room?

The issue I found was transferring open mouth morphs from M4 to M4 Reduced Resolution. I ended up using the open mouth morphs a kind friend had hand built for me to use on my V4 RR.