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.