Oh, @simon, I love you!! I didn’t associate the problem with these settings because the error occurred recently, and I only touched there at the beginning of the installation, about a month ago. I suppose I don’t visit these pages (TOS, FAQ…) often enough!
Thank you very much!
In any case, I think the explanation you can find below the small window where you can enter those values is not very clear. At least, not in Spanish. See:
It says something like: If you have a document for (privacy policy, TOS, or FAQ) hosted anywhere, put its URL here… But wouldn’t it be better to add “but leave it empty if this URL is at your Discourse installation” or something similar? I assume that by writing just a few more letters, it would be clearer for other clumsy guys—or girls—like me in the future… 
It’s an idea… Thanks! Gracias!