Hi r ana,
I checked jBPM 4.1 and this problem seems caused in org.jbpm.pvm.internal.type.converter.SerializableToBytesConverter.convert(). The exception information that you provided was not enough, so I can't go further.
First, I suggest you use the latest version of jBPM 4, jBPM 4.3.
Then if the problem cannot be solved, please show us the full exception stack trace. You'd better give us a testcase, so we could reproduce this problem and find out a solution.
Thank you.