User who is trying to Log on to Exchange Admin Console is logged in to ...
Workaround. To work around this issue, use the URL of the Client Access server in the site that has the user mailbox to log on to EAC. For users who do not have the mailbox, use the URL of the Client Access server that hosts arbitration mailbox SystemMailbox {bb558c35-97f1-4cb9-8ff7-d53741dc928c}.