We we have this error with JasperReport 3.5.0.
We have this error with some reports in our application.
The cause seems to be a class loader problem:
We use a URLClassLoader in our application. (the archives are loaded dynamically)
To bypass the error, we use Thread.currentThread().setContextClassLoader(null)) before the call to JasperFillManager.fillReport (...), but Lucian (lucianc) says that's can have adverse effects.
Since the test case was difficult to extract from our sources, we send you a Apache Ant task that produce the same error.