Hi @radin.mattia ,
In case it’s an issue with any transient backend service incident, I’d suggest trying to restart Outlook application and see if there’s any difference.
If the issue persists, based on my experience, you could refer to the blog below to try to disable FAST search through the registry.
How Outlook 2016 utilizes Exchange Server 2016 FAST Search - Microsoft Community Hub
Important: Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.
Some info that might help:
Assuming FAST Search is not disabled by policy, Outlook initially submits a query to Exchange on-premises using FAST Search. If FAST Search results are returned in a timely manner (about 5 seconds), these are displayed. But, if the FAST Search timeout is reached before a server response, then a message indicating that We're having trouble fetching results from the server..., along with the link "Let's look on your computer instead." If you click this link, then WDS search results are displayed.
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