JSON from table query as input doesn't work

Hi, I 've run in a weird (at least for me) problem. I query a table and make the result an input for an custom function. This custom function creates an csv file from the given json.

When I just copy the query result json as input in the Run & Debug from the custom function, it works perfect. When Iet it run via the function stack of my API, I get an empty (null) csv file.

This is the main stack.

Here do I set the input:

This is the custom function:

Any idea what I'm doing wrong? Do I need an additional filter for the query result?

Thank you in advance,

Stefan