You add a shared mailbox to your user mailbox as "MultiEx." If both mailboxes are located in Microsoft Exchange Server On-Premises, the search in the shared mailbox works as expected. After the Message Bus Exchange (MBX) is migrated to Exchange Online, the search in the shared MBX mailbox fails, and you receive the following error message:
We are having trouble fetching results from the server... Let's look on your computer instead.
This limitation forces you to migrate user mailboxes and their associated shared mailboxes at the same time.
To fix this issue, install Cumulative Update 13 or a later cumulative update for Exchange Server 2019.