Shotcut 17.06.01 portable does not start

I downloaded the latest version of shotcut portable and unzipped the file onto my external disk. When I started shotcut.exe I first got the error message “Qt error when loading platform plugins for windows“. I googled and found out that I need to copy the qwindows.dll into the folder platforms. Now shotcut starts to run and I see the sign “ shotcut loading plugins“ for about one second. Then the sign disappears and nothing is happening anymore. What am I doing wrong? Thanks for your help in advance.