@koahnig Hi, thank you for your answer!
I think it works now.
To be honest I don't really know why it didn't work. I tried a lot, also turning off firewall and load online and offline installer from different pages for case, that the .exe is corrupted.
Last thing I did is to check the regedit and delete all Qt*-data in HKEY_(...)\SOFTWARE. Its working now. The problem was probably because of unclean deinstallation of previous version.
