Thanks, so the Certify challenge server seems to be working correctly as it can resolve the challenge response itself, however when Let’s Encrypt tried to resolve the URL (such as http://centro-espanol.org/.well-known/acme-challenge/2K0AUxIgZKnuwiAgQ3rx8t2rY9gXXbct8oUPukDjZ2w) it got an http redirect (303). from IIS. This shouldn’t happen if this is the Certify server that’s responding.
Are you load balancing internally or otherwise proxying the requests via another server or could your public DNS settings be wrong? It just seems as if the server that responded (78.94.117.234) was not the same server that Certify is running on and it then got passed to IIS to handle. You should also reconsider using an http 303 direct and consider 301 or 302 instead as it looks like Let’s Encrypt doesn’t handled 303 (so it can’t then fallback to using IIS for http validation).