@eeeee is right, it's pretty much an arms race between those who want to get the file, and those who don't want the file downloaded.
Keep in mind that if a browser is viewing the file, then it has de facto already downloaded the file. The "save as" functionality is mostly a convenience feature for you to re-locate the downloaded file elsewhere.
@projectmoon@forum.agnos.is hmm... could be a bug in the user migration code path.
Can you run your forum in dev mode (./nodebb dev) and try to load some of the categories here in your forum, and let me know if you see any errors in the console?