Microsoft is rolling out a fix for the Outlook search issues to all affected environments. Microsoft announced today that a fix will be deployed to all Outlook clients. The issue is caused by a bug in the authentication process which leads to the misrouting of search requests, inadvertently causing some of them to fail. Microsoft did not say what caused the influx of search issues, but user reports say they appeared right after applying two problematic updates that brought the version of the Outlook desktop client to 2004 12730.20236.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-fix-for-outlook-search-failures-coming-later-today/