I believe we will need more information to reproduce this behavior.
This is a server-side .NET exception, so it shouldn't matter that your client machine is running Linux. Do you encounter the same exception when accessing your website using Firefox 3.0 from a Windows computer (XP or Vista)? What kind of activity is the user performing on your page when the exception occurs? If the same activity is performed, does the exception occur consistently, or only intermittently?