Hello Florian!
Maybe Vienna Ensemble has been blacklisted by Sibelius at some point. Please try to rescan the plugins in the Sibelius menu Play/Setup/Audio Engine Options. Then restart your computer.
If this doesn't help, I would try delete the "Plogue Engine" folder from Users/Username/Library/Application Support/Avid/Sibelius/PlogueEngine.
The folder Users/Username/Library is hidden by default. To get access, go to the folder Users/Username and select Go to folder with the Finder. Then enter "Library".
After deleting the "Plogue Engine" folder, I would restart the computer.
I hope this helps.
Best regards,
Andi
Vienna Symphonic Library