Hi Peter Smillie,
Welcome to our forum!
Does the error occur on Outlook for Web? If so, it is suggested please go to office.com and sign in there. After you sign in there click the dots in the top left navigation to go to Outlook:
If this does not work, since your issue seems to be especially related to a Microsoft Entra authentication and authorization AADSTS error code, please understand that there is a limited amount of knowledge available about some aspects of the scenarios. It is recommended that you post your concerns in the appropriate community at Microsoft Entra ID - Microsoft Q&A for more professional assistance.
Hope it helps.
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