error ".pst file is in use by another application"

[Guest]

I have two users (out of 35) who repeatedly get the ".pst file is in use by
another application" error message when trying to open their Outlook 2002
Personal Folders. I have tried running the Inbox Repair Tool (after closing
Outlook) and I get the same message that the pst is in use by another
application. The pst files are stored on a network server and there are no
backup applications running when this error occurs. The only way I can "fix"
it is to copy the pst file to another location, open Outlook, delete the old
location in Mail Setup, then add the new location. Unfortunately, this
solution doesn't stick and I end up doing the same process within a couple of
months.

Any ideas?