Thanks, Kaibaseto. It seems you're right. Although the download page shows a large number of seeders and leachers, a fresh download of Ragnarok did not go through, and I got the same error message as before.

NarutoMaster, I deleted the .torrent file, and also the .sfv file (in the folder with the episode files). BitTorrent scanned the files, but stopped at 99.9% Done and then returned the same error as before.

So, altough your method got BitTorrent to rescan the download to process it. It seems that, in this case, the trouble lay in an inability contact with the Tracker.

Thanks for your help, I'll try to seed Ragnarok later.