This message is typically produced when FileMaker attempts to open a file that is not hosted by FileMaker but has already been opened by another user or via a different user account. How fileMaker attempts to open the file as well as how or if it is currently hosted on your network could be factors.
Are you linking a table in one file to a table in the MES SPECIAL ED 12_13.fp7 file at the time?
If no, are there table occurrences that refer to a table in this file and you are getting this message when you first open Manage | database?
Where is this file? On you computer or on a server? Is it currently hosted by FileMaker Pro or FileMaker server when you see this message?
Open Manage | External Data Sources and look for any external data sources for this file that are listed there. What do you find?