In a Microsoft Exchange Server 2019 or Exchange Server 2016 hybrid environment, you deploy modern public folders on Exchange on-premises. When an online mailbox that is the owner of an on-premises public folder (for example, onpremPF) signs in to Outlook, and tries to assign permissions to an on-premises or online mailbox for the on-premises public folder (onpremPF), the permissions can’t be applied.
To fix this issue, install one of the following updates:
For Exchange Server 2019, install the Cumulative Update 5 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019.For Exchange Server 2016, install the Cumulative Update 16 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
Learn about the terminology that Microsoft uses to describe software updates.