Hi Mabel,
thanks for the answer.
As you can see on the first screenshot below, the form (on the left hand side) is fairly complex and cannot be easily transformed into an Excel Table without altering the visual structure. So I thought I could create a very small table (on the right hand side), - that I successfully filled with data via step 4 of the flow - and reference this table in the corresponding cells of the Form:
- Cell E10 and E11 reference the "Date" column of the table on the right.
- Cell D12 references the "Reason" column
In order to take the screenshot, I opened the Excel table manually and then the references are correctly processed. Thus the "23.08.2018" and "Bootcamp Mittagessen Test" being present in the form.

This second screenshot below is the PDF I sucessfully produced at the end of the flow. As you can see, the table on the right is correctly filled, but the references on the left are not refreshed.
This is the problem I am facing right now. The references only seem to be processed when I open the workbook and not when I "just" modify the data in the background with a flow and then export it as a PDF.

Does it makes more sense?
Here's the PDF creation:

I don't think it has to do with formatting of the cells since when I open the workbook manually, all the references are correctly processed. I added a delay before creating the PDF but it doesn't seem to have any effect...