JOB command fails with Embedded Python

It sounds strange though it's embarrassing.

I have a class with ClassMethods written in ISOS and Embedded Python.

The ClassMethods written in ISOS start in the Background as usual. No problems

But my 

ClassMethod setPy() [ Language = python ]
{
import iris, datetime, time
- - -

Just fails the JOB command with (even 15 sec) timeout and $ZC=0
This is not funny

My actual workaround:
I created a ClassMethod in ISOS, that does an elegant JobStart
and this ClassMethod only runs the Embedded Python ClassMethod.

I'm not amused and hope there is just an undocumented/unknown param missing

I have a feeling about what's going on. But it's not for public discussion.