Solved it myself. The issue was with Windows automatically blocking unknown .exe files from unknown sources. The downloaded setup.exe files were (b)locked by Windows 2016 :-/
Thanks @juraci note that licensed customers are welcome to use our priority support helpdesk by emailing support {at} certifytheweb.com
Regarding the blocking by Server 2016 we’ve not heard of that before so you should review that your system is able to contact the standard microsoft SmartScreen api services as our setup file is digitally signed.