We are here! For me, this error occurred after update to CU11. I can not open OWA after update, but ECP was works good.This was occurring because the computer object was added to a group that is denying this key/token ms-Exch-EPI-Token-Serialization As Microsoft said, the following group have the token denied by default. Domain Admins Schema Admins Enterprise Admins Organization Management If you run gpresult /scope computer /r and check the groups that the computer object was added. To fix the problem, you need to remove the computer object from that groups. Reference https://support.microsoft.com/en-us/help/2898571/error-message-in-ems-eac-ecp-owa-or-outlook-on-the-web-in-exchange-ser
