Hi Werner,
The default of Namespace USER is to be mapped as Interoperability (aka ENSEMBLE) - enabled.
If you create a new namespace, deselect the Interoperability-Enabled checkbox. 

You may of course also just delete Namespace USER and create a new one.
OR remove the mapping directly from iris.cpf. 

Big THANKS to @YURI MARX GOMES  (getting red cheeks) 
You confirm, that my intention to pass along the accumulated experience works.
And I'm glad I can contribute to this community which I had in mind (~ 15years back)
but couldn't realize myself. 
So I have also to thank @Evgeny Shvarov and his team that made my "dream" true and successful.

typo in first (mailed) version:  DO $SYSTEM.SQL.Shell()   
dot instead of colon.

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