Also the 'c' attribute (compression) makes it fail.
In my situation, I was trying to move files between 2 shared folders on Synology DSM (on the same btrfs volume), except one of them was marked for compression. Rather than take compression off the whole shared folder, I targeted the parent folder with "chattr -c".
I wouldn't have though this would prevent the CoW, but it worked fine after that.
Stephen Kestle