Assuming you really enter the same account name and password every time, I'd suspect a problem with your keyboard. I have never seen a functioning computer behave inconsistently - usually it's the human's fault. Keep in mind that passwords are case-sensitive.
I noticed when I enter the accounts and privileges, the password changes to a 20-digit string (which I can't see because it's starred out). Why is FileMaker changing the password?
The password is not changed, but the true length of password is being obscured for security.