If the file is on a server, make sure that it has not been set to be “available offline”. If it has been set up for this, then uncheck that setting on the file (in the directory listing) to remove it from the offline caching. Then reboot the computer where the user is experiencing the issue. This is particularly relevant if the user has noticed that they get this error when opening the file from one computer but do NOT get this error when using the same user account on another computer.