Shut the PC all the way off.
Remove the scanner from the 'loop' completely - unplug its cable from the keyboard and system. Connect the keyboard normally.
Power on the PC normally.
With the PC running, connect the scanner to the PS/2 keyboard port and the keyboard to the other end of the scanner's wire.
Scan Factory Default from page 6.
Reboot the PC - try scanning normally.
This applies to both the WCS3900 and WCS3905.