Supplied key (null) is not a RSAPrivateKey instance error in the Server Settings / Server Certificates screen

keytool insists on storing full certificate chain when importing certificate into the keystore, therefore it is sometimes necessary to import a CA root into the certificate keystore (not only into the truststore).

After importing certficate for the key, all additional certificates (intermediary CAs, root CAs, etc.) should be deleted from the keystore

using "keytool -delete", otherwise a "Supplied key (null) is not a RSAPrivateKey instance" exception will be displayed in the Server Certificates screen.

Exception itself can be fixed easily: