Hi,
the easiest way is to debug the call and have a look at the error. Or just print the error to the console, it's either report.errors or ReportBroError exception depending on where it fails.
In the latest version there is a setting menuShowDebug for the
designer. If you set it to true then there is a menu button where you
can output the whole report definition to the javascript console. Copy
the whole report definition and upload it somewhere (e.g. pastebin).
Then I can have a look at this.