Symptoms
Consider the following scenario:
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You have installed Microsoft Exchange Server 2019 Cumulative Update 9 (CU9) or Microsoft Exchange Server 2016 Cumulative Update 20 (CU20).
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You add a mailbox-enabled user to the "Recipient Management" role group, or you assign the "Mail Recipient Creation" role to the user.
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You log in to Exchange Admin Center (EAC) as this user.
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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.
Resolution
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.
References
Learn about the terminology that Microsoft uses to describe software updates.
