HDD formatted NTFS, as expected, but even FAT32 should be able to handle anything under 4GB. I tried downloading to a different HDD - same error.
It just finished downloading and interestingly, the file is perfectly fine even though it's bigger than the KSP zip. I won't try the Linux approach until the weekend, if anything goes wrong I don't want to be in a hurry to fix it: I need my PC for work, too. But I tried something else: I logged into the KSP store through my phone, downloaded the file and copied it over to the PC - it extracts without throwing errors.
I'm not sure if I should be happy or not... On one hand, that means that my OS handles zips just fine (as it always did in the past), on the other hand it might indicate that my PC isn't communicating properly with my router anymore. Some additional info: My internet connection is over the 4G mobile network. The router is actually just an glorified antenna with a SIM inside. https://i.imgur.com/YLno3UA.jpg Reception on my PC is through a WiFi dongle in a USB port.