Using older versions of Outlook for Mac in business environments
Try opening your profile folder in terminal: Outlook 15 Profiles
Run extended attribute list: ls -leo@
If you see: [Main Profile 3] com.apple.quarantine 22 the folder has been quarantined for some reason.
Remove this attribute: xattr -dr com.apple.quarantine 'Main Profile 3'.
Search immediately worked after this attribute was removed.