If you are having trouble similar to the issues described in KB305217,
the symptoms of which are:

When Internet Explorer version 5.5 Service Pack 1 or later tries to POST data,
GET data or set up an HTTPS connection with the connect command,
Internet Explorer generates an error message that indicates that the page could not be displayed.
This problem does not occur in Internet Explorer 5.5.

then you might find that turning off keep-alive on your webserver may fix the problem. It helped for one of our clients.
Some further info can be found in
this newsthread.

Yes, this is a fairly serious bug. No, Microsoft are not distributing a fix.