Case #
You are trying to logon to either Exchange Online admin center at https://outlook.office.com/ecp or Outlook Web App (OWA) at https://outlook.office.com/ from any modern browser but you fall into a loop of endless redirects.
Solution #
Microsoft 365 or Office 365 subscriptions provide a set of admin roles which can be assigned to users in a tenant by using the Microsoft 365 admin center or Powershell. Each admin role maps to common business functions and gives people in the tenant permissions to do specific tasks in the admin centers. The endless loop redirection issue with Exchange Online admin center and Outlook Web App (OWA) occurs when a global administrator (company administrator) user has also explicitly been assigned the service administrator for the Exchange Online services.
To resolve the redirections loop issue, you must remove any Exchange Online related roles from the affected global administrator user or assign an Exchange Online service administrator role to a user who is not also a global administrator.
Sources #
https://docs.microsoft.com/en-us/microsoft-365/admin/add-users/about-admin-roles?view=o365-worldwide
