Consider the following scenario:
You have installed Microsoft Exchange Server 2019 Cumulative Update 9 (CU9) or Microsoft Exchange Server 2016 Cumulative Update 20 (CU20).
You add a mailbox-enabled user to the "Recipient Management" role group, or you assign the "Mail Recipient Creation" role to the user.
You log in to Exchange Admin Center (EAC) as this user.
Under Recipients > Mailboxes, you select the plus sign, and then you select that user's mailbox.
In this scenario, you receive a "500 - Unexpected Error" error message, and you don't see an option to create a new mailbox.
To fix this issue, install the following cumulative update, as appropriate:
Cumulative Update 10 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019
Cumulative Update 21 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
Learn about the terminology that Microsoft uses to describe software updates.