"Oops - something went wrong" when trying to sign in to a Microsoft account

I have the solution!

The issue is that you have a Family Safety adult setup on that machine that is in the same group of the Microsoft accounts your trying to associate to a local account!

When you go to the account settings, you will notice that it shows the option to let the other Microsoft accounts within the Family Safety group to log in if you enable it.

I'm 100% sure that when you try to convert a local account to a Microsoft account that is already listed as a Family member, it will fail to convert it as it looks like it's already associated to your computer even though you never enabled it!

I removed the Microsoft account that was already on my machine from the Family group. I then logged into that account and went into the account settings. I saw the other Family members on that page for a short time and then they disappear.

I then logged out of that Microsoft account and logged into my son's local account. I was able to switch it to Microsoft successfully.

I then logged into my wife's local account and switched it to Microsoft successfully.

Again..

The bug is, if the Microsoft account to want to associate to a local account shows up in any bodies Family list in the Account settings, it will fail to work.. You have to make sure NONE of the current users has that name showing up for it to work. Those users are cached! So even if you remove them, you have to log back into that account and pull up the account page so it refreshes the list before it will work!