What is the exact syntax of your command line? On my system, and I would expect on a "normal" Access system, when you change the user name, you should need to enter a password of some sort.
You said that "Admin" is the name you get when you use the CurrentUser() function. In truth, Access security is always turned on. Unless you change the default security settings, everyone in the world gets the same default Admin user with no password as their default user. That's why everyone can openly share files with one another - even though Access security is turned on.
Anyway, somehow, the user name you are passing in your command line is not being processed correctly by Access (obviously). Let's take a look at your command line syntax to see if that's an issue.