I solved the problem on my Outlook.com. See the post above for my specific situation where I postulated that I caused the problem by creating a conflicting Office 365 Enterprise test account. That turns out to be true. Here is how I fixed my problem.
This is very situation specific but I hope the logic of my approach will help others to find a solution and for Microsoft to help:
1. I deleted the DNS entry that Office 365 Enterprise ("O365E") setup had instructed me to make on my DNS server.
2. In O365E I created a new Global Administrator account in the default domain (non-conflicting).
3. Logging in as the new administrator, I first changed the default domain from my (conflicting) domain to the O365E default domain.
4. I then changed the email address of the other user account to the default domain. I then deleted that account which, to that point had created a conflict with an Office 365 Home Premium ("O365HP") account.
4. I then deleted the conflicting domain so that now the only remaining domain was the original O365E default (non-conflicting) and the only remaining user account used a non-conflicting email address and had never used a conflicting email address.
After all of this, I still had a O365E test account and a O365HP paid account, but there were no conflicts in any of the email addresses or domain names. I then logged out of O365E and successfully logged onto Outlook.com and the account information associated
with my O365HP account.
My conclusion is that the whole problem was caused by either a domain name conflict or an email naming conflict or both. That was implied in early post from Mary Grace above and I think that my experience proves it. As I said, my solution is specific to
my situation but I hope that it helps y'all too.
JEC