There have been a number of reports recently that the Instant Search feature in Outlook 2016 and Outlook 365 stopped working after installing the December 2018 Security Update KB4471321. Initially, this issue appears to be on Windows Server 2016 when running as a Terminal Server or Remote Desktop Server. There have been a few reports of the issue appearing for Windows 10 users as well.
Microsoft has acknowledged the issue and has stated that they are working on a resolution and will have an update in an upcoming release.
How To Fix Outlook Instant Search Stops Working After Installing December 2018 Security Update KB4471321
The good news is that there is a workaround that people have reported success with that doesn’t involve uninstalling the December 2018 Security Update KB4471321.
Click on Start and type Command Prompt or CMD
Right-click Command Prompt and click Run As Administrator and Allow it to run
In the Command Prompt screen, type sfc /scannow and press Enter
Wait for the scan to complete then restart the computer
Outlook Instant Search should work after the restart. We will update this article when Microsoft has provided a permanent fix for this issue.