This is my configuration:

My parent links are the following:

And the problem is that when I try to make the transition (I'm on TW-9), I get this exception:
An unknown exception occured executing Validator com.fca.jira.plugins.workflowToolbox.AnyIssueValidator@7dfdf768: root cause: java.lang.StringBuffer cannot be cast to java.lang.String
I have had a look to the logs, but I don't find anything explanatory.
Any idea?
Thanks.
Sergio