Database won't run on certain machines

All,Having a peculiar problem. I have created a database that creates email accounts using the RegisterNewUser method. It works fine on several machines, with the exception of two. These two machines are running windows XP and one has version 6.0.3 and the other 6.5. When they try to press my “submit” button, it keeps asking for the cert.id file, and when they locate it, it always tells them the id file they are trying to create already exists. What it does after that is create the id file and it places it in their local Notes directory. They have proper access to the ACL and everything, and I am not sure what else to try.