Scanner Locked Error
Scanner Lock -- a possible quicker fix
Instead of removing the entire glass plate, just remove the narrow glass strip to the far left. To do this, open the scanner top revealing the glass, and remove the philips screw centered on the left edge. With a thin flat-head screwdriver or even a good set of fingernails, you can then pry out the thin U-shaped plastic piece holding the glass strip in place.
When you remove the glass strip, you now have (narrow but sufficient) access to the scanner pulley and belt. Pull the spring-loaded pulley back towards your right to get some slack on the belt, then remove the belt from the pulley.
This frees up the scanner head, which you can now slide manually (by pulling gently on one end of the belt and giving slack on the other) to a center position well away from the scanner lock. When I first tried, I didn't move the scanner head far enough -- to the middle or beyond seems sufficient.
Push the scan lock to the full lock and then the full unlock position, to make sure it is now free.
Using fingers, screwdriver, and/or pliers, re-seat the belt on the pulley, once again using the freedom to move the spring-loaded pulley a bit to the right.
Replace the glass strip, plastic cover, and screw.
Power on the scanner. The error message should now be gone, and the scanner head should slide back to its resting position on startup (unless something is caught underneath, which was apparently the case for some who had this problem but was not the case for me). If the first power cycle doesn't do the trick, another power cycle might be helpful.
This approach eliminates the need to remove the scanner lid, control panel, and scanner glass.