I still can't get in with either address. ... I have no idea why.
I believe you. However, the forum's access log says that you've never tried to get in using either
http://www.dsboards.com/ or
http://www.dsboards.com :-/
A few people had a very similar problem trying to get into the archive. I had no idea why it came up then, just like I have no idea why you're getting it now. A few of them were able to upgrade their browsers and that corrected the problem for them. Sadly, that's not an option for you because you already have the latest Mac version of IE.
Apparently there must be something different about the way the Mac version reports environment variables - and, unfortunately I don't currently have access to a Mac to see if I can figure out what might be going on.
About the only thing I can suggest is that if you still have Netscape 6, or if you have the hard disk space to install it again (you can have more than one working version installed), try using it until the issues with Netscape 7 get resolved.
Sorry - I wish I could be of more help...