Reporting module Groovy Error

I have a legacy server 1.11.6 on which i cannot run groovy based reports. It throws this error. Failed to parse template script (your template may contain an error or be trying to use expressions not currently supported): Failed to create Script instance for class: class SimpleTemplateScript1. Reason: java.lang.ClassCastException: SimpleTemplateScript1 cannot be cast to groovy.lang.GroovyObject.

Any pointer on how to resolve this?

Can you share the server side log?