@Richard Hine Thank you for reaching out to us, regarding this error/authentication loop - "More information required" with error code 50074, seems mfa for your account was never registered.
Can you share the correlation id & time stamp at the time of issue to know the exact cause of this login failure ( either conditional access or security defaults on the tenant ) causing this issue.
Have you tried going to this page - aka.ms/mysecurityinfo (https://support.microsoft.com/en-us/account-billing/set-up-security-info-from-a-sign-in-page-28180870-c256-4ebf-8bd7-5335571bf9a8) to try alternate methods of mfa and see if it helps to access your account.
If not reach out to me over email on azcommunity [at] microsoft [dot] com referencing this issue with a subject line "ATTN:Givary" so that we connect offline over teams and get this resolved for you.
Let me know if you have any further questions, feel free to post back.