Alrighty. It sounds like you've got a firewall program blocking it. In my experience, it's usually something like Norton Internet Security blocking the requests. Something is preventing it from hitting your site, otherwise if it was just timing out (the other semi-common error), it would take about 15 seconds to die.
Try entering another URL like www.dkpsystem.com, or demo.dkpsystem.com, and see the result you get there.
Are you not running any firewall programs (I'm not referring to a router, or hardware firewall, just some internet security programs that block outgoing requests).
The key thing to note is that the program hasn't changed, and if the result is popping up instantly, it means there's something external blocking the request. The only other alternative is the address is entered incorrectly, but it sounds like you have the address right.
I am having this exact same problem with my guilds dkp.
It has not worked for probably a month now.
concordis.dkpsystem.com
Have tried putting the http:// in front of it, have tried all caps, not working.
I just tried the other persons website and it allowed me to download his just fine.
Unlike his, my error is taking around 5-10 seconds to come up, its not immediate. So for the past month, to month and a half, I have been having to download the file and manually update it, so we can continue to use the mod.
I can upload the snapshots just fine, but the grss downloader will simply not work for me.
I am having this exact same problem with my guilds dkp.
I have the solution to your problem and will get it deployed with the next version. You'll actually have to download the newest version straight from the website, rather than using the autodownloader, as you actually need a new autodownloader.