(only applies when using Windows Vista/Server 2009 or newer)
!MC5 stores all its data files in a subfolder inside windows' common appdata folder (typically C:\ProgramData\Bartsch Software\MC5). During the installation this folder is created and full access is granted for all users, to make sure all users use the same configuration, conversion files, message archives, and so on. The common appdata folder is monitored Windows' file virtualization. When a user lacking the required rights tries to write to this folder Windows File Virtualization steps in. Instead of denying access, the file get written to the users virtual store (and subsequently read from it, when accessed by the same user). As a result different users see e.g. different versions of a conversion, configuration or database file. When using !MC5 with different users (e.g. when using the add on Windows Service) this can cause all sorts of trouble.
Windows File Virtualization should not be deactivated and is in effect while UAC is enabled. Disabling UAC is not recommended! Instead make sure all (required) users have full access to !MC5s data folder. As already mentioned, file permissions are set accordingly during setup but might be changed when migrating to another machine or due to security considerations.
Also, if any files got virtualized, those have to be moved to their original destination. Be careful when doing so and only replace those original files with their virtualized counterparts if you are sure, the virtualized one is newer!
!MC5s default data folder is (typically) located at: C:\ProgramData\Bartsch Software\MC5 (ProgramData is a hidden folder)
Windows Virtual Store is (typically) located at: C:\Users\UserName\AppData\Local\VirtualStore (AppData is a hidden folder)
When starting !MC5 the program tests if the folder C:\Users\UserName\AppData\Local\VirtualStore\ProgramData\Bartsch Software exists. If so, it displays a warning. To remove that warning, delete the folder (but only after applying the steps mentioned above!).