Hi Thomas,
please use currently the qnap installer from roononnas.org (version 2020-07-15). The QNAP app center version still has an issue in the installation routine and will hopefully soon be replaced with the new installer.
You can download version 2020-07-15 on the very top of the page (first section, when the text starts). Then click the manually install button in the app center of your QNAP administration:
