VSS is trying to open a non-existent file, or a file to which the path is incorrect. VSS attempts to resolve file paths relative to the location of the SRCSAFE.INI file located in the server installation folder.
This error has the following possible causes and solution:
The Journal_File variable in the SRCSAFE.INI file is set to a location that is unavailable, such as a drive mapping that has been disconnected.
You are attempting to show differences on a file whose working directory is a floppy drive, and there is no disk in the drive.
In either case VSS must be able to locate the file it needs to complete the operation.