Tried it now, and it works normally.
Note that, the activation servers are many, and the activate.esellerate.net name will point to different (random) servers, you might try clearing your browser cache and try again, perhaps your browser is stuck into a server that is momentarily down at the moment.
UPDATE:We just got word from Esellerate they had a problem with their DNS configuration today, it only lasted 3 hours and it's now fixed, but it's possible your ISP takes more time to update the DNS with the fixed configuration, so this might have affected the activation, but it might work if you try later on.
It usually takes more time in Europe for ISP to update their DNS, one way to fix this might be using a different DNS setting instead of the default one from your ISP, such as Google DNS
https://developers.google.com/speed/public-dns/docs/using?hl=itor OpenDNS:
http://use.opendns.com/I always use Google DNS myself, so I haven't noticed a problem until an email from Esellerate arrived explaining it.