Outlook, Office365, authentication & autodiscovery problem

I'm trying to add a Microsoft365 email account to a new  Outlook profile (installed & up to date from Microsoft365). The default Outlook profile does not sync for office365 email address a@domain1.com. I tried making a new Outlook profile to test.

I successfully added the problem address to the new profile2 - a@domain1.com

When I  go to add another address, a@domain2.com,  it fails

The Outlook autoconfiguration test runs, but does not prompt me for the  domain2.com user's login details. It shows:

http status=401   (that's access denied I think)

autodiscover to https://outlook.office365.com/autodiscover/autodiscover.xml failed.

Testing autoconfiguration with a 3rd office365 email address (x@domain3.com, say) on the problem pc, I'm prompted to log in, and it succeeds.

Repeating the failed domain2.com autoconfiguration test on a different computer, 1) I'm prompted for the username & password, and 2) it works fine.

So there is a problem related to authenticating for this particular domain2 address on this particular laptop . It has been used before, so there may be remnants somewhere. 

I have removed everything related to this account and Microsoft/Office logins in Credential manager. 

I'm not sure what else to try. Uninstalling and reinstalling Office did not help.

Thank you, whoever replies!