When following the AWS S3 example https://github.com/jboss-fuse/camel-kafka-connector-examples/tree/0.7.1-redhat/aws2-s3/aws2-s3-sink-with-aggregation#set-the-aws-credential-as-secret-optional and adding the secret to the S2I config just as in the example then the example connector configuration leads to an incorrect secret file.
For me the correct configuration was ${file:/opt/kafka/external-configuration/aws-credentials.properties:accessKey}