Hi
I have more or less the same problem. Have tried to create a Kafka setup using Docker and SASL_PLAINTEXT with SCRAM-SHA-256. It works for me if I only start a single broker, but if I start a 3 node cluster, I get the same error.
It works if I change the controller to use PLAINTEXT/PLAIN.
listener.security.protocol.map=CONTROLLER:PLAINTEXT,LOCAL:SASL_PLAINTEXT,BROKER:SASL_PLAINTEXT,DOCKERHOST:SASL_PLAINTEXT,EXTERNAL:SASL_PLAINTEXT
sasl.enabled.mechanisms=SCRAM-SHA-256
sasl.mechanism.inter.broker.protocol=SCRAM-SHA-256
sasl.mechanism.controller.protocol=PLAIN