The RSA key container could not be opened.

Hi all,
Here is my problem.
My web.comfig file in the webserver is encrypted . I want to do a change to the config file. While I'm trying to decrypt  from the command  prompt on the server using following command. 
"aspnet_regiis -pd "appSettings" -app "/AppName" I endup with the following error message.
Failed to decrypt using provider 'CustomProvider'. Error message from the provider: The RSA key container could not be opened. 
The RSA key container could not be opened. Failed!
The same command has been working in other environments except in my servers.  Also I'm using web forms and I tried the same in both the servers. I have gone through all websites but no use.
Thanks in advance for your help.
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