www.swi-prolog.org is not responding.
I checked, it is not just me. 
Odd. I’d guess that a CDN offers close to 100% uptime. Anyway, all seems working (again).
us.swi-prolog.org was not responding for a short time around the time @EricGT reported a problem, but that shouldn’t affect www.swi-prolog.org as long as eu.swi-prolog.org is still up.
It feels like a DNS problem. Nagios keeps reporting that it cannot resolve different hostnames in swi-prolog.org, even when our internet connection is lightly loaded.
The ISP says 
Interruption NS-01 AuroraDNS cluster
2 hours ago
We’re currently experiencing issues with one of our AuroraDNS clusters (ns01). Our technicians are investigating the problem and we’re working very hard to resolve the issues as quickly as possible. Our sincere apologies for the inconvenience.
The problems seem to be resolved.