When attempting to open an AVID DNxHD file in QuickTime 10, you are asked for a MediaSeal Authentication Password, which you enter correctly. The authentication is successful, however QuickTime reports that the file is not a valid QuickTime file.
DNxHD is a codec specifically created by AVID for use in AVID Applications such as Pro Tools.
AVID DNxHD formats are not supported by the latest versions of Apple’s QuickTime player.
In order to view DNxHD files, you will need to install the AVID Codec LE and use a Media Player that supports the DNxHD codecs e.g. QuickTime 7
Download link to Codec:
Download link to QuickTime 7: