One of my machines, a remote, will not connect to Gpugrid.net at all. All the others are fine, so it's not the server...
This particular machine has AT&T as it's isp.
I think, but I am not sure, that they are blocking your site. I can't upload or download WU's or even connect thru google or another projects link. not even to the forum.

Are you aware of any previous issues such as this? Is there a work around?

I doubt that AT&T is doing the blocking. All of my machines connect through the bellsouth.net part of AT&T, and the one with a suitable Nvidia card is not blocked; and at least the website is not blocked for the others. One thing I'd check if whether the blocked machine uses an antivirus program that can block certain sites, and happens to have added the GPUGRID site to its blocked list.

Another thing I'd check is whether that machine uses a dynamic DNS listing, and your central machine hasn't caught up with the latest change in what IP address it uses.

Also, is the blocked machine inside a company that blocks access from their machines to some external web sites?

Can you even do a remote login into the blocked machine and check if BOINC is running there?

Note I've seen some reports that the remote login method included in some versions of Windows is known to interfere with GPU use on at least one end of the connection, for as long as that connection lasts.

I've seen a proxy method of obtaining workunits and sending back the results mentioned in a way that suggests it might be a suitable workaround, but without enough detail for me to tell if it's suitable.

Something GPUGRID may want to check: Whether their normal connection method makes any use of port 25, which many ISPs block for reaching any machine other than their email server.