Please can you show your problem in the form of screenshots? I am not sure the problem you are facing to try and reproduce it on my side.
But it only happens on one machine
I am not sure why you are entering the username STEFOX443. If you have reset the Administrator password the username would be admin
Every time I open PiL on my laptop, I am prompted to log in to Playit Live again
However, when I try to do so I am told my username or password is invalid. I know this isn't the case, having written it down after the third admin reset.
What am I doing wrong?