ConcurrentModificationException in GraphicsRenderer() -> ExpandoMetaClass -> ClassInfo

A lot of concurrent running tasks that use the same task to generate a chart with GraphicsBuilder.
The involved variables in this part are either static (read-only) or local. As shown in the stack-trace the
Exception is thrown in the constructor of GraphicsRenderer()

def img = new GraphicsRenderer().render(width, height) {
//...
}

I will install 1.6.2 today on the server and report if the errors occure there too.

Find the stack-trace attached.