The HotRod client's keySet() operation throws ClassCastException due to the following reason:
When Encoder2x.scala in its writeResponse() method decodes the operation as BulkGetKeysResponse, it runs a Map/Reduce job that returns a set of keys in the whole cluster.
The operation returns a set of "unmarshalled" (cos we're in compatibility mode) entries. However, Scala infers the type of individual entries as "Bytes" which is an alias for Array[Byte].

As a result, when this iterator from this key set is retrieved, it is not possible to iterate through the entries because Scala automatically tries to convert each (unmarshalled) entry into a byte array, which results in the exception.
This line results in throwing CCE:
https://github.com/infinispan/infinispan/blob/master/server/hotrod/src/main/scala/org/infinispan/server/hotrod/Encoder2x.scala#L231