Here is another thing to try. An Event holds your actual video clips. The Project holds the path to the event clip and the in and out points (the addresses of the frames from the clip that you use in the project). If one of these entries gets corrupted, then you will get an error.
Open the problem project and Select a clip. Then press Command-A (or Edit/Select All), and then Command-C, (or Edit/Copy).
Go back to the Project Library View and start a brand new project. Paste your clips. (you will only have your video clips and not any audio.) To paste, type Command-V or Edit/Paste.
At this point just start from one end or the other and delete one clip at a time and try to export.
You should keep getting the error until suddenly it works.
At that point you know the clip you just deleted was the problem clip.
Go back to your original project, find the problem clip, delete it and replace it with a fresh copy of the clip you need.
Another thing to try. Right click on your Event file and see if it offers to "Optimize Media". If so, optimizing media should help.