After you install the July 2021 security update for Microsoft Exchange Server 2019 and 2016, users can’t sign in to their mailboxes by using Outlook on the web (OWA) or Exchange Control Panel (ECP).
The issue occurs if the server authorization certificate is either expired or not present.
To fix this issue, install the following updates, as appropriate:
Cumulative Update 12 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019
Cumulative Update 23 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016