This error is usually due to a reason that they are incompatible with RealPlayer. Here are the options for you:
1. Make sure you have the latest version of RealPlayer. You can install it from www.real.com. (If you purchased RealPlayer v.18, please click this link to install RealPlayer)
2. You can use RealConverter to convert the existing MP4 file to a different video format available within RealConverter.
If the issue persists, please let us know.