In my test environment I wanted to add another Mailbox server to my DAG and upon running the install it failed with the following error:
“Upgrade the discovery mailboxes to R5 version, this will fix the RecipientDisplayType property of the discovery mailbox which was wrong in R4”
First I ran Setup /PrepareAD again but this didn’t resolve the issue. Running a search in Active Directory I found 2x Discovery Search User accounts, as they were not needed the accounts were deleted and the setup retried, all successful.
Hope it helps.