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This also occurs with Internet Explorer, using certain combinations of Security Level, proxy and firewall settings, certain ISPs, and may be dependant on which way the wind is blowing.
No, seriously, this is a problem that is reported from time to time. No-one knows the exact combination of settings that fail; suffice to say that the submission then goes through without problem if the user trys Netscape, Mozilla, Opera, or basically anything that isn't Internet Explorer.