What may cause 'Following URL address is not valid' when publishing InfoPath form to MOSS and to cope with it?

Question

Hi, we have a rather old MOSS installation with SP2 installed, which quite recently ceased to include the MicrosoftSharePointTeamServices header in replies to Protocol Discovery OPTIONS HTTP requests. This causes problems like Excel 2003 complaining of being unable to connect to SharePoint site to synchronize lists, etc. The Excel problem can be worked around by fixing the type of the corresponding HKCU\Software\Microsoft\Office\1x.0\Common\Internet\Server Cache entry to be 5 or 6. But the same workaround doesn't get InfoPath back on track when publishing templates to Form Libraries, while at the same time the template can be saved as to the DocLib\Forms\template.xsn location.

Any ideas as to what the culprit may be and how to get over the problem?