Bruce,
Did you ever manage to get the files to play?
I had this same problem but it does not appear to be limited to QT. Files do not play in camera, or VLC, or MPEG Streamclip, or Canon Image browser. I was recordeing on a 64gb sandisk extreme pro w/udma 6, 90 mb/s. The last 9 clips from an evening of recording do not play at all. I tried using sandisk rescue pro to recover data but had NO luck.
If you found a way to make the files useable, please post here!
Thanks.