This error generally means that there's something on your timeline that FCPX doesn't like such as a corrupt clip, unusual codec, a DRM-protected audio track, CMYK image, etc.
From Crash Analyzer:
-50 (QuickTime)
This error occurs when there is corrupt or invalid media in your timeline. Check that the codec used by the file is recommended for use in your editing application.
Suggested Actions
- If you are using Final Cut Pro 7 and your sequence codec is set to a long-GOP codec such as HDV, H.264, XDCAM, AVCHD or MPEG-2, set your sequence codec to ProRes.
- For best results, convert all long-GOP media to ProRes.
- Make sure all images are RGB, not CMYK.
- Ensure all images are less than 4000 pixels in width and height.
- Use Corrupt Clip Finder to locate the corrupt clip.
- After the corrupt file is located, use QT Repair or Media Salvage to fix it.
- Make sure your files do not have Digital Rights Management (DRM) that prevent them being played outside of their intended application. Some files purchased from the iTunes Store will have DRM.
- Make sure you are using AIFF or WAV audio files.
- This can sometimes be caused by third-party codecs. Use System Toolkit to temporarily disable them.
More Information
System description for error -50: Error in user parameter list
Report generated by Crash Analyzer in Pro Maintenance Tools
