After an update of the Exchange Server there was a problem that the search no longer worked. The index showed no problems, but no results were displayed in OWA and Outlook:
The problem is due to "symbolic links" in the registry, which may no longer be correct after an update. However, there is a script in the Technet which sets the links correctly again:
http://gallery.technet.microsoft.com/scriptcenter/16afe88b-6c6d-49a3-8b03-2a2ece27f61a
In principle, only the Exchange search services are terminated and the index service is re-registered. Since the search is not working anyway, this can be done during operation.
After executing the script, the search works perfectly again immediately.