A root certificate error occurs when using a certificate previously used on a different model of camera, or used on the same camera for FTP transfer with Secure Protocol set to On (FTPS transfer) before the system software was updated.

To help prevent security risks, a gradual increase of the security level causes a root certificate error to be displayed when the camera cannot check whether or not the certificate in use may have been revoked.
Prepare a root certificate for which the camera can check revocation.

Otherwise, you can enable a connection by setting MENU → Network → FTP Transfer → FTP Transfer Func. → Server Setting → "Select a server" → Destination Settings → Root Certificate Error → "Connect", but do this at your own discretion, because security may not be ensured.