Do you have the actual error message or number?
As a start I would run the permissions and library repair from Aperture 3: Troubleshooting Basics
If that doesn't fix it do a disk repair from Disk Utility.
Finally you could remove the file. To locate it do:
In Finder find the aperture library (Aperture Library.aplibrary)
Right click the library and select Show Package Contents
Select the Thumbnails/10rfEeaET9Ckt7jpxyGV5s folder
remove the AP.Thumbnails file
If you still have problems post back