I have now hopefully fixed the Webtorrent problem.
The issue was simple: I had seeded the torrent with the test page with a command line client (webtorrent-cli) that uses node.js. But this client however can only connect to regular Torrent clients (e.g. bitTorrent or webtorrent-desktop), not to browser-based clients like the one the Slimweb gateway uses.
To seed the web pages / torrents that should be accessible with the Gateway, one must use the webtorrent-hybrid client (see package here). This one has much more dependencies (about 200 MB), but it works.
The Slimweb test page
(Everybody that wants to test the Gateway with his/her own web pages and wants to seed the torrent files from a VPS should download webtorrent-hybrid. The desktop version of Webtorrent should work fine to seed, and you could even use http://instant.io to seed your web page. I'll soon provide a newbie-friendly guide to the website or the Wiki.)
*Unfortunately, I made a test inscription from the address that contained the original inscription of the torrent hash of the test page. I must sync the wallet with that address and then inscribe the torrent hash again, then the test page will work again.