Hi I'm beginning to be quite desperate. I already tried every single solution proposed on the support pages, but out of 5 separate download there where always multiple corrupted files.
I tried downloading via different browsers (Chrome, Firefox) as well as GOG Downloader, which is supposed to be able to correct corrupt chunks but it doesn't help!

Any suggestions how to get the installation to work?

I've had the same problem. It worked on the third download though - I decided not to use the GOG downloader.

you may have a bad stick of ram, this will result in corrupt downloads