MS Project SP3 Problem

After upgrading Microsoft Project to Service Pack 3 from Service Pact 2, some files that were created in SP2 will not open. We get the following error:

An unexpected error occured during command execution.

Try the following:

Verify that all argument names and values are correct and are of correct type.

You may have run out of memory. To free up available memory, close programs, projects, or windows that your aren’t using.

The files open in Project 2003 SP2 and Project 2007 with no problems. If I save the file as a Project 98 file, then project 2003 SP3 can open it. There are no passwords set on the files.
Does anyone know what the problem is?