If this certificate contains a private key, move this certificate to the DPM recovery certificates collection for recovering ...

1) If this certificate contains a private key, move this certificate to the DPM recovery certificates collection for recovering data in future, and replace this certificate with a valid certificate in the DPM backup certificates collection to continue further backups.
2) If this certificate does not contain a private key, replace this certificate with a valid certificate in the DPM backup certificates collection to continue further backups.
See "How to Encrypt Data in a Protection Group " in DPM Help for more information about certificates.