Since you were able to run rtmp://s2pchzxmtymn2k.cloudfront.net/cf … sintel.mp4 locally, I would assume there was no problem with the media file. Also, since you could play other media files in Amazon, then I would assume firewall or outbound connection was not an issue, either.
I am looking at the possibility that the flash media driver may be failing on some media files. Since we cannot test different players in exactly the same scenario, can we compare the version of Flash (not FlowPlayer) installed on your local machine and the one in the Amazon instance? If they are different, can we install the version of Flash used in your local machine to the Amazon instance?
Let me know how it goes.