I’m going to suggest a few different things, as this error can indicate basically anything.
-What is most likely is you have a corrupt file somewhere in your project. One way of checking is stringing out all your materials in a sequence and trying to render. The clip that won’t render is corrupt. You will need to get rid of it and bring it back into the project. I suggest converting from raw again if it’s video or re-exporting an aif from QT7 if it’s audio. In some way, you need to refresh the file before bringing it back in.
-Quit FCP and delete all of the files in “Render Files.” (If you don’t want to re-render your sequences, try the other options first.) Be sure to empty your trash before re-opening FCP.
-Quit FCP and trash your preferences. This is a great catch-all for FCP errors. Your preferences can be found here: [Your system username]>Library>Preferences>com.apple.FinalCutPro.plist Empty your trash and reopen FCP.
-Restarting your whole system often helps with these mysterious errors as well. FCP will bug out if the computer or program has been up and running too long.