The 2 Java classes IReportCompiler.java and ReportQueryDialog.java have a section of code that is almost identical.
Starting with "IReportConnection connection ="
In IReportCompiler however, the connections getSpecialParameters method is called:
hm = connection.getSpecialParameters( hm );
It doesn't make this call in ReportQueryDialog.java.
For a custom iReport connection this means that a required Parameter value required to access data is not available
at the point a connection to the data source is made, which causes a connection exception.