The reason is that the constructor chosen by the ThrowableSerializer does not accept a null cause, and CordaThrowables are stripped of their cause field by RPC proxies.
Exception observed during testing was:
I'm going to assume this is coverd by commit
28:e44b6c6f4 : CashException is unable to derisalize with AMQP through RPC. (fix). (#3274)
Which seems to have been commited with the wrong JIRA in the commit header