Symptoms
After you install the Cumulative Update 10 (CU10) for Microsoft Exchange Server 2019 or Cumulative Update 21 (CU21) for Microsoft Exchange Server 2016, you can’t edit the custom attributes by using the Exchange admin center (EAC). Even after you make changes in the /ecp/UsersGroups/EditMailbox.aspx window and select OK, changes are not saved. When this error occurs, errors are not reported or logged.
Resolution
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
