Old post, but here's the correct solution for anyone who may have this problem in the future. I don't think that always leaving a disc in the tray is a solution, just a temporary fix.
My parents had the identical unit with the same problem. They fixed it by replacing it, and let the 3400 sit and collect dust.
I got sick of this so I set out to fix it today. EASY fix. Should take the novice about 45 minutes, and expert closer to 15 minutes.
This writeup is for educational purposes only, with varying skill levels in the world, I'm not going to be responsible for any damage you cause to your DVD player, yourself, your TV, your neighbor, or your cat.
T-10 Torx screwdriver
P-2 Phillips screwdriver
And a small slotted screwdriver, knife etc.. something to bend tabs back to unsnap plastic pieces.
Make sure the unit is unplugged first.
Remove 7 T-10 screws from cover (2 on each side, 3 on back)
Locate metal bracket over DVD unit.
Remove 2 T-10 screws holding bracket to bottom.
Remove 3 T-10 screws holding bracket to front.
Be careful you don't break the mount tabs, and don't touch any circuitry!
Remove 2 P-2 screws from black cover over DVD unit. Using your weapon of choice, CAREFULLY pry the tabs that are holding the black cover onto the DVD unit. 1 tab on each side, near the front of the machine.
DO NOT TOUCH THE LASER LENS!
With the removed cover in hand, locate 3 tabs that snap 2 white plastic circle halves together, Seperate the halves. Remove 2 washers. Notice one is a standard washer, and one is a strong magnet.
REVERSE their locations. Stack the magnets on the side that comes in contact with the DVD. Place the strong magnet on first, then place the normal washer on top.
I believe the cause of the "BLOCKED" message is this magnet being too strong/the opening mechanism being too weak. You need a magnetic field
to center the white circle onto the base that the disc sits on, but too strong of a field causes the BLOCKED message to appear. Lessen the magnetic field and everyone is happy.
Reassembly is the reverse of removal.