I have this when running JDG 6.1.0.ER6 with JGroups 3.2.4

08:31:41,460 ERROR [org.jgroups.blocks.RequestCorrelator] failed sending rsp: return value (SuccessfulResponse{responseValue=null} ) is not serializable

which is of course not the cause of the error.
This message is logged when any Throwable is caught
https://github.com/belaban/JGroups/blob/26b7c60d511baf199674b45d3d402dfbbc667e9e/src/org/jgroups/blocks/RequestCorrelator.java#L509

in my case it was once NullPointerException and once InterruptedException probably caused by caches and externalizers shutting down.

I'd recommend rewording of the message if the exception is not NotSerializableException or the like to avoid confusion.