I'm posting this again, maybe you've missed it some weeks ago in the other thread:
I had the same issue. In my case I guess the AD-blocker ("Disconnect") was the problem. After adding the muse wallet site to the "whitelist" ("wallet.museblockchain.com" - where you create your account) and creating another new account AFTERWARDS everything went fine...
But I still can't log in to the very first account I created "successfully" before and get the same "user not found" error. So please check if any of your add-ons is blocking the site in any way. Then create a new account and try it again.