Why does removing a custom domain from Azure AD fail?

I created a new Azure Active Directory and added our real domain to it (verified through TXT record in DNS). When I was done playing around with it, I created another one that I intended to use for production purposes. When I tried to add our domain to the new AD instance, it complained that the name is already in use and that I need to remove it from the previous instance first. But when I try to do so, I get the following unhelpful message despite having all of the three prerequisites (Users, Groups, Applications) checked off:

"Deletion of ' ****.com' failed. Please try again."

I have tried again, many times over a few days. A similarly unhelpful message appears in the Notifications.

Does anyone have any ideas?

Edit:

Here is more information on what I did:

1. Signed into portal.azure.com with a personal account.
2. Created a new Azure Active Directory
3. Switched current directory to it (had to refresh the portal for the option to appear)
4. Added a new Global Administrator user with onmicrosoft.com at the end to that directory
5. Signed out and signed in with the new AD account
6. Deleted the personal account from the directory
7. Added a custom domain and verified it
8. Added another user with our custom domain at the end
9. Signed into the new account just to see that it works
10. Created another AD instance (don't remember from which account)
11. Switched to that directory
12. Added a new Global Administrator user with onmicrosoft.com at the end
13. Signed in with the new account
14. Deleted the old account
15. Started the process for adding a custom domain
16. Changed our TXT DNS record to match the new AD
17. Tried to verify the domain, which failed because it was already in use
18. Signed into an account from the first AD
19. Tried to remove the custom domain, which failed because there was a user using it
20. Removed all other users (from the onmicrosoft.com account)
21. Tried to remove the custom domain again, which failed with the unhelpful message above

Edit 2:

The reason I was doing all this is that we needed to migrate from the old Partner Membership Center, which uses personal accounts, to the new Partner Center, which uses work (AD) accounts. All the steps above were taken before I had started the transition process from PMC. I have now switched to the new Center and used the second AD with an onmicrosoft.com account. But I would like to create a new Global Administrator with our real domain in its name. I did not use the first AD because it had the word "experimental" in its default domain.

Edit 3:

The personal account's email address is at our own domain, the same one I was adding/removing to/from the two AD instances. Even on my first sign-in on portal.azure.com, before I had created any AD and associated the domain with it, I was being asked whether I intended to use my personal or work account. But then the password reset tool said that such work account did not exist.

Maybe this is somehow related to why I cannot disassociate the domain now.