Thank you both! I tried with the Slicer-4.13.0-2020. It is working, it creates the DICOM RT file.
When I import it to Eclipse, it just gives some errors:
S002: (F) 2020/11/12 21:39:27.841 [21960] Mandatory element contains no data (Element: (0008,1155) ‘Referenced SOP Instance UID’)
Not sure, if this is something that I can edit in 3DSlicer