Hi Mario

This sounds like a problem that I have come across before. 

The admin sub-navigation block is setup to be a locked (single instance) block. This block had been deleted from a page thereby removing it from all pages. It appears that like the administration block it has to be set to appear on every page otherwise the navigation for many admin pages does not appear. 

Adding the block back is easy, go to any admin page where the block is missing, and add the block in the normal way. (It appears in the list of available blocks.) Set it to show on all pages, save the settings.