NOTE: once you remove the key, take a good digital pic of the key contacts before you do anything. Then try the instructions below. Look for anything that might be different from any other key.
ONLY: attempt this if you have contacted the support team at Classic for authorization. Failure to do so before attempting may void your warranty.
For black keys you have to remove the adjoining white key first.
1) Unhook the spring from the back of the key. It unhooks at the bottom of the spring.
2) Use something to gently pry the rear of the key out of the white hinge-housing (at the pivot point near the springs)
3) Slide the key forward to get past the front hook
gently massage the rubber boot underneath the key. (it should have two recessed areas. The contacts are in there. If something is stuck under the pad, the contact can't release. Massaging the rubber can release whatever is stuck.
You can have Hauptwerk running so that you can test for the sticking note.
Reverse the order to re-assemble.