According to the documentations, when doing a REST PUT, "Any existing data will be replaced", but when the cache is configured to store "application/x-java-object" (unserialized objects) and the object does not implement the equals method, the replace always fails and the server returns HTTP 412 (precondition failed).

This is due to the fact that internally the REST server first retrieves the existing value, and then does a conditional replace with this value that never matches.