Besides, the HW4 link I renamed to 'Hauptwerk for Sibelius' starts the HW4 with completely wrong sounds... When pressing keys, the correct keys on screen seems pressed, but I hear completely different notes (or none at all)
I don't think this has anything to do with the 'Sibelius connector', but even resetting the organ did not solve it.
The regular startup of HW4 does work correctly, so maybe the cache is broken? I would love to get this problem solved too.
It shouldn't ever be possible for Hauptwerk's sample set data caches to be invalid (unless you overrode Hauptwerk's normal behaviour using the 'Design tools | Load organ (with design options)' functions). To verify you can force a sample set data cache to be regenerated by loading an organ with 'Organ | Load organ, adjusting ...' and just clicking OK.
If you launch that Hauptwerk configuration and temporarily untick *all* entries on both the 'MIDI IN ports' and 'MIDI OUT ports' tabs on the 'General settings | MIDI ports' screen, and then play Hauptwerk by clicking on its virtual keys, do you then hear the expected pipe sounds? (If so, then presumably the problem is somehow related to MIDI messages being remapped/echoed via Sibelius.)