Issue is on the Microsoft Live side, they are putting too much information in the request header based upon all the different tenants you're using. This eventually creates an issue where it exceeds the allowable length of the header & causes the 400.

You can confirm this by using an Incognito/In Private window or logging in from a different browser.

Best way to workaround the issue is to clear your browser cookies & cache.

The same situation can also arise when a user belongs to "too many" Entra/Active Directory groups and the amount of GUIDs going into the header exceeds the maximum length -- see https://learn.microsoft.com/en-us/troubleshoot/developer/webapps/iis/www-authentication-authorization/http-bad-request-response-kerberos