I found this procedure here.
There is a locking hook (see below, red arrow pointing to it) inside SA-21 (and SA-30) adapters which prevents you from removing these adapters from Nikon LS-5000 (and LS-4000) scanners while the film strip is still inside.
During the normal operation, this hook engages when you insert the film and disengages (see below) when the film is ejected.
If something goes wrong and that hook gets stuck in an engaged position, you cannot remove the SA-21 (and SA-30) adapter from the scanner. Usually closing the application and turning the power OFF and then back ON helps. If not, follow this procedure. As always, YOU ARE DOING THIS AT YOUR OWN RISK!
The steps 1), 2) and 3) posted here are for LS-5000 (and LS-50) scanner. For LS-40 and LS-4000 first perform steps 1) and 2) from the LS-4000 disassembly procedure, then proceed to step 4) below.
1) Unplug the power & USB (Firewire) cable and remove adaptors. Set the scanner on a clean area (you do not need to set the scanner upside down).
2) Remove these 6 screws on the back of the unit. 2 at the top, two at the bottom, and two in the middle, I’ve screwed the out already on this image.
3) Pull out the rubber pads as shown by a red arrow above. There are 4 pads in the corners on the bottom of the scanner, and they cover 4 screws. Remove these 4 screws and slide the cover off. Turn the scanner top side up (normal position).
4) Manually turn the white plastic gear (my finger on it in the above photo) until you can see the retaining hook inside SA-21 the opening in the optical head (indicated by a red arrow in the photo above).
5) Push the retaining hook IN using a small flat screwdriver. This should take very little force. You should be able to remove the adapter out of the scanner now.
6) Put everything back together in reverse order. (You may consider checking the mirror while the scanner is open).