OK, thanks for that and thanks for my high school french class.
I'm going to go out on a limb here and guess that you've already eliminate the actual error by assuring that the file is not infact opened by another user and therefore locked to prevent simultaneous editing.
Believe it or not, I've seen this problem with Microsoft Access databases (no matter how you set locking) and the solution seemed to be avoiding drive mapping when calling the network resource.
Perhaps try creating new shortcuts to the folder containing the documents on the user's desktop by using the full UNC path to the resource something like Link Removed just as a test to see if it cures the issue. I also found that often opening the application and then...