What is priv_sock_get_cmd error?



 I am running vsftpd server on CentOS 5.
 Sometimes, I am getting the error: "500 OOPS: priv_sock_get_cmd".
 This error doesn't happen always. So, I don't have the exact steps to reproduce the problem.
 Everything will be working fine and all of a sudden, I get this error!

 So, I wanted to know, what are the cases in which this error can happen?
 And what is the meaning of this error?
 Are there any solutions or work arounds to solve this problem?