Is the username in the message the actual users name and the file is being opened using the default login provided in the file setup?
If so then you can ignore this. What is happening is that FileMaker is trying to open the file with the users name, this fails and then the file is opened with the default account.
This generates a warning, but you don't need to worry about unless the user is not getting access to the file.
That's exactly what happens, and the only thing that is anoying is the amount of Warnings in the log it generates.
The log is intended to show you what is going on, and it is. I recommend ignoring this. You can limit the size of the log so that it doesn't take up to much space. Just don't go to small so you don't miss important things.