If you receive an error from Kontakt Player 2 saying "The patch could not be loaded (unknown error)" it means you're trying to load instruments (patches) from a newer VDL library into an older version of Kontakt Player.
For example, Virtual Drumline library version 2.5.5 requires Kontakt Player 4.2.4 or greater, so attempting to load its patches into Kontakt Player 2 will yield this result.
To fix this, you should replace any activated instances of Kontakt Player 2 with a more current version of Kontakt. If your host program requires you to manually load your instruments, you'll need to re-load them as well.
Sibelius users: Here's a quick reference of which versions of Kontakt are compatible with Sibelius' auto-load feature.