I had a problem with this codec too, when trying to open a video file attached to an email.
When I try to download the file, and click 'open', the file is saved to a temporary folder and realplayer tries to open and play the file. this is when I get the codec error.
I discovered by accident that if I save the file in my video folder and open it from there, I can watch the video with no error. I have done this with two different video files to verify.
The codec may have something to do with opening the file or location in temporary file. Hope that helps.
