More information:

I installed Firefox and tried that and the problem disappeared. So I tried clearing the cache in IE6 and that made no difference. But clearing cookies in IE6 DID apparently fix the problem. So it appears it was a cookie problem after all (maybe I should change the title of this thread?).

So, the question is, how was the bad cookie created, and why wasn't the error recoverable? Sadly I don't think I can recover the offending cookie now but if the problem recurs I will post it.