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Hi Albert,
Thanks for posting back.
Please use the private window of the browser to access Outlook for Web and test again. If the issue still exists, the issue is likely caused by the backend. I would like to suggest you create a support ticket about this issue via Office 365 Admin Center > Support. That support team there will have the correct channel to help you investigate more and find what exactly reason has caused this issue from the backend.
Appreciate your understanding and patient.
Regards,
George