- I tried the same torrents in µTorrent on the same computer, and that works without a problem, so it doesn't seem to be a connection problem, and the torrents certainly have lots of seeders.
- I saw a suggestion to go in the Advanced settings and set "Network interface" to "Any interface", and "Optional IP address to bind to" to "All addresses". Both were already set that way.
- I don't know when this problem started: I don't download torrents all that frequently. I think it still worked like a month ago. I also don't know if I updated qBittorrent in the meantime (I use Chocolatey to update a lot of software semi-automatically).