error 15466 An error during decryption

I have a stored procedure that runs xp_cmdshell with a user with sysadmin right.(it was created by someone else long time ago). For security purposes I need to remove the sysadmin right from this user. So I created a proxy account and gave execute privilege to this user on the xp_cmdshell (practically followed the steps how to run xp_cmdshell with a non sysadmin user ). But when I run the stored procedure I am getting the below error: > error number 15466 severity 16 errorline 1 An error occurred during decryption. I use SQL Server 2008 What could be the problem?