Ok it appears to be two things going on.
For item 1) it is a corrupted complex object. MatterHackers tech support says it is due to a mesh defect. Not sure how that came about as the object was created using add and subtract parts of MC.
For item, 2) Apparently I was not waiting long enough for the subtract to complete.
The hourglass and progress bar for Subtract is WAYYYYYYYYYYYYY TOO teeny!
Thanks to Tech support for spotting and point the progress bar issue out but MatterHackers should make it bigger. Seriously.