friedrich.hayek: Why the heck does it not work? Do I need to rename part 6? To what?
Assuming GOG hasn't changed them, check if you have a clean download (
-6.bin in particular):
$ md5sum setup_the_witcher_enhanced_edition_2.0.0.12.exe
66ffe865f34e71ef2beb961748873459 setup_the_witcher_enhanced_edition_2.0.0.12.exe
$ md5sum setup_the_witcher_enhanced_edition_2.0.0.12-1.bin
6d24dcb24f4776889e03715044ca8da0 setup_the_witcher_enhanced_edition_2.0.0.12-1.bin
$ md5sum setup_the_witcher_enhanced_edition_2.0.0.12-2.bin
3060962cd3ef2ec68a9c02fdeb5ce839 setup_the_witcher_enhanced_edition_2.0.0.12-2.bin
$ md5sum setup_the_witcher_enhanced_edition_2.0.0.12-3.bin
602a920d5e2c05437f70c5600911aca8 setup_the_witcher_enhanced_edition_2.0.0.12-3.bin
$ md5sum setup_the_witcher_enhanced_edition_2.0.0.12-4.bin
296605a9b5e7acba6a59cd17b98a84c6 setup_the_witcher_enhanced_edition_2.0.0.12-4.bin
$ md5sum setup_the_witcher_enhanced_edition_2.0.0.12-5.bin
a820dc0f09afefead9e06a9c37491c1b setup_the_witcher_enhanced_edition_2.0.0.12-5.bin
$ md5sum setup_the_witcher_enhanced_edition_2.0.0.12-6.bin
e5261e0eff49b83f48a384ece3050106 setup_the_witcher_enhanced_edition_2.0.0.12-6.bin