Database opened by another user

Users are receiving this message: "This database currently being used by someone else. In order to share Notes databases, all users must use a Notes server instead of a file server."

The server is unable to access databases for replication. Users do not appear to have the database opened from the file server. I also checked the backup software and it is skipping over this file because it is seeing it as an open file. Is there any other place I might look for resolution?
I am pretty sure that you are getting this message because the database directory on the Domino server is mapped as a shared drive on someone's computer. If so, this is not allowed. Only the Domino server software should access the NSF files on its disks.