You must have put your false teeth in backwards, Bob. I upgraded via system reports (it told me LM21.1 was available) and it went without drama or fuss.....and still is. I have been giving it a hammering all day. Rock Solid. Not a toe out of place.
Where did your download come from?....I have seen a hell of a lot of 'dodgy' downloads floating around the net over the past few days.
Did you allow sufficient ram and space in Virtual box?
Mint says there are "Known Issues" well why don't they fix them ? Instead of telling people to try workarounds.