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Internet Explorer - 500 internal server error, even on localhost

IE becomes unable to 'see' your plone server via localhost or any URI, but other browsers see the server fine.

IE suddenly stops being able to connect to URL where plone server lives, even using localhost, IP and port#'s. (win32 2.0.5 stable, on XP sp2 with all the latest everythings, running a LAN cable connection)

Netscape and Firefox see the server just fine. The connection fails (timeout) or returns an http 500 error.

Google search on this turns up a LOT of similar incidents in about every server config you can imagine, even MS servers, and as many different solutions too.

Here is the one that worked, for me.

Solution: (in IE)
Tools >> Internet Options >> connections tab >> LAN settings >> uncheck the auto detect feature

You likely need to restart the browser, I did a full reboot to be certain.