I'm in version 4.01 (I think I need to nudge my provider about enabling the things necessary for upgrade 'cause I haven't succeeded in any attempts...)
I tried to upgrade my profile and am informed that my user name must be all lower case. Guess this is a new thing because it was acceptable when I put it in there the first time.
So I changed it to auntjames instead of auntJames and am informed that's already taken.
Really? AuntJames.net wasn't even taken as a web page....
When I clicked to get more information https://docs.moodle.org/400/en/error/moodle/usernamelowercase welp, nope, that doesn't exist.
Not a big deal -- when I get to the point where yea, I really do want to update I'll think of something.
From a distance the case is clear. You can't have AuntJames, AuntJames, auntJames, AuntJAMES or any such thing because auntjames is there. Moodle saves the username in lower case!
I know, it is not nice. The prev. or prev. prev. Moodle must have stopped auntJames entering. But people toptoe.

I know, it is not nice. The prev. or prev. prev. Moodle must have stopped auntJames entering. But people toptoe.
Think we are confused ... AuntJames.net is a domain name ... not a user name .... and it could be used for another moodle instance on the same server as your resourceroom moodle instance.
Matter of fact, in DNS, AuntJames.net is pointed to the same IP address as the resourceroom domain.
And the resourceroom site is a 4.0. The more info about this link went to /400/ which is 4.0 of moodle.
Since they are both on the same server ... same environment ... so if in the moodle that is working, one went to Site Admin -> Server -> Environment check ... and update the component, you could the drop down pick list for moodle version to see what is required to run that version you chose in the pick list.
SoS', Ken
Agree with Visvanath - all usernames must be unique and must be lower case. If you previously had auntjames with any kind of capital letter/upper case it would have insisted on lower case so now you can't change it to all lower case. This isn't anything new -usernames have always been lower case and unique. I