Now it is the second time it happens that the search cannot return any results. This hickup is rare but it happens. To solve it I had to follow these steps:

1. Stop the Timer Service
2. Clear the configuration cache
   1. Find in \ProgramData\Microsoft\SharePoint\Config the folder where the file cache.ini exists
   2. Delete every file from this folder EXCEPT cache.ini
   3. Open cache.ini, delete the content and put ’1′ (without the quotes) in it and save the file
3. Restart the Timer Service
4. Index reset
5. Full crawl

Source: ITIDea. The linked blog post saved my afternoon today. Thank you, Anita Boerboom.