Thank you for your post.
I don't have an answer for you, as I have never encountered this.
Does this occur immediately when you select a file? Or, does it appear the file is loading before you get the error message? Are there any On Open scripts with the files? That is, you can have a script execute when a file is opened. If so, do any of these scripts attempt to open another database file? If so, does this other database reside on another server that possibly that user doesn't have access to? Is the Login information stored in a FileMaker file? If that user tries to login with an account that another user is successful on another machine, does this work? I'm not there to look at your setup (and I'm sure it is fine), but I need to find out as much information possible to narrow down some possible causes.
I just had another set of eyes look at the setup and they spotted the issue. It turns out that the user had added an extra space at the end of the IP address when he added the host to favorites. It still allowed him to see the files, but errored when he tried to open anything. I wish I could break my users of the bad habit of hitting that spacebar! I've had to add the Auto Enter Calc/Trim to all the text fields in the databases. Thank goodness for that option! :smileyhappy: