are your workstations linked in some way?
when i worked on a show with 4-7 systems attached to an X-Serve Raid, we would get this.
if any of the systems crashed, the remaining systems couldn't save their FCP projects to the X-Serve.
solution was to save to the desktop,
then copy that project file back to the X-Serve, where the project files were living
quit FCP, and when you open it again, all should work fine. (or did we have to shut down and re-boot? sorry, cant remember)
nick