EC2 Instance launch with error saying that "we can't signin to your account" after changed the Username

Hello All,
One of the EC2 instance we created suddenly occurred the error saying that "we can't signin to your account" and sign me out again and again. Any solution what may cause the issue? Is it because we change the username from Administrator to something else like myadmin that cause this issue?

We change the username as we don't want hacker attack our instance easily as they maybe guess the username like admin or Administrator.

Please find the attached image below for the error message I saw when I launched the instance.