for future reference: i couldnt fix the problem, but using the open source "free download manager" does the job as a workaround.
plus i download much faster now. it seems to me this problem is not directly related to itch, since i observed slow download speeds at other web pages in the past few days (but no expiring downloads). i guess itch is just very sensitive in that regard, so it breaks the connection easily
i couldnt really figure it out. some people said it may be my network configuration, so i played around with that for hours, reset, followed a bunch of "guides" that fixed similar problems. nothing really made a difference, so im just gonna use the download manager now.
probably just windows being a microsoft product or something. maybe its time for a clean reinstall *cries in pain*