Root certificate error may occur if a certificate used on a different camera model or the same camera for FTP transfer with Secure Protocol set to On before updating the system software

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a root certificate error to be displayed when the camera can't check whether or not the certificate in use may have been revoked.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