@Former Member 

Make sure you have configured "nifi.registry.security.needClientAuth=false".
When not set it defaults to true.

NeedClientAuth=true tells NiFi that in the TLS handshake it will "require" client to present a client side certificate.  If one is not presented, the connection will just close and NiFi-Registry will never try any other authentication method.

This property must be set to false in order for NiFi-Registry to support any authentication method other than TLS.

Hope this gets you going,

Matt