Finally got to it...
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As a side note, these Macs that do and don't work are on the same network, right?No; the various Macs which I've tried to get into the website are all unrelated, their only commonality being that they belong to people to whom I can entrust the passwords...no networks to be found anywhere.
Try this, just for kicks:
On one of the Macs where Safari doesn't work, go to Safari's preferences, click Advanced, and then click the Show Develop menu in menu bar (assuming you don't already have it turned on).
Then click Develop->User Agent->Firefox 3.5.3
Then try to go to the Web page. If it works, then what's happening is that the script is written to return a phony 404 error if it does not recognize the browser.
I forgot to mention that I tried that in Safari 3.2.3 with no joy, but I didn't get around to trying in Safari 4.0.5 until you posted...
No joy with IE 8.0 or Firefox 3.5.3.
Edit: It's just kinda odd that Panther's Safari can get in; is it possible that Safari has changed in some way that's caused this issue?