The client entered the USER command to log in to the server. The server is using the client certificate that is passed to the server when a TLS protected session was established to authenticate the user. The client certificate is already defined for another process or the certificate does not meet the required format.
The login to the user ID is rejected.
Contact the system programmer.
Find the certificate in the security product and determine whether it is a valid certificate that is registered (associated) with the user ID that is logging in. The required format for the certificate is defined by the __certificate() function in the z/OS XL C/C++ Runtime Library Reference.