BlockLegacyAuthentication fails for Organization Policy because of BackendRehydrationModule implementation

Applies To
Exchange Server 2019

Symptoms

When authentication policies that use BlockLegacyAuthentication assign the policy to the organization, legitimate Modern Authentication requests fail and return an “HTTP Status 401” error message.

Resolution

To fix this issue, install Cumulative Update 13 or a later cumulative update for Exchange Server 2019.