External System key error while decrypting the encrypted document

I am getting decryption error with the following message. Can any one guide me in resolving this issue

I have followed the steps described by Nick Vigilante in the knowledge base External System Key Error thrown when executing a plugin

This plug-in [xxxxxxxxxxxxx] is not registered to access secured values for the given external system key [[externalSystemKey=xxxxxx, fieldKey=password, usePerUser=false]]. Check the external systems plug-ins list in the Administration Console.