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Thanks for the answer - but I guess that would be the answer for a 3. question :)
The error I get seems to say that I'm not authenticating properly - I get the same error if using an altered MY_ACCESS_KEY_ID.
I used the ROLE_ARN as param in the call in order to avoid the possible non-inheriting in the credentials file,
but in a final code version, I'd like to have that too only in the credentials file.