This problem delayed my project a long time before I sat down to fix it. Finally I found a solution. What happened was that I tried to play the .MOV clip that caused the error message ”Codec could not be opened” in Final Cut Pro. The sound was ok but no video.
QuickTime than suggested me to download a codec called ”XviD_Codec Component 1.0 alpha”. So I downloaded it and followed the installation instructions. I put the codec inside the QuickTime folder in the Library among the other codes. Note! You must not put the codec inside the QuickTime Library folder within the systems folder.
I restarted QuickTime player and now video was fine. Then I restarted FCP and wonders above all wonders, video was perfect in FCP. No more ”Codec could not be opened” when I rendered it.