So Mu stopped working for me.
It does work while editing in the Max engine, but various IDs and such lose scope while under the tender mercies of Live itself.
So I suspect something has changed in Live 8.3.4, as this is the only change to my system that could possibly affect Mu to take place within the timeframe we're talking about (within the last week).
I guess consider this a cross between a RFC to confirm if 8.3.4 is the culprit and a PSA that 8.3.4 is (probably) the culprit.