Hi @Kenny M ,
Welcome to our forum.
Typically, this issue occurs if a computer object is added to a group that is denied the ms-Exch-EPI-Token-Serialization user right.
To resolve this issue, please remove the computer object from the restricted group.
For more details, please refer to: error-occur-ems-eac-owa
If the above suggestions do not work, the following similar threads hope to help you.
exchange-2013-unable-to-access-ecp-403-sorry-access-denied
exchange-2016-ecp-error-403-after-login.html
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