PrimeGrid server was inaccessible for about 30 minutes because the network at the datacenter where PrimeGrid and BOINCstats are hosted was down. The datacenter technicians seem to have brought the network back up, but we have no idea if it's going to stay up (the support phones of the hosting provider are down).
Update 18:11 UTC: it seems that the power was cut to the building; the servers were forcibly rebooted. I'm checking for stuck system processes now.
Update 18:15 UTC: validator was stuck; it's fixed now. I think everything is running smooth now.