TYPO3 Core

Not sure what has changed in the way that the Extenstion manager retrieves the repository list for TER2 but it no longer works when the server is behind an ISA server.
I have a Windows and a Linux server behind an ISA firewall with rules setup for both to have access to any site with any protocol, but neither can access TER2 extensions through the Extension manager.
My old 3.8 install with TER works fine, but the 4.0RC1 installs with TER2 does not.

Error from WAMP 4.0RC1 install on a Win2003 server:
Warning: file_get_contents(http://repositories.typo3.org/mirrors.xml.gz) [function.file-get-contents]: failed to open stream: HTTP request failed! HTTP/1.1 500 ( Not implemented ) in C:\Program Files\typo3\htdocs\typo3wamp\t3lib\class.t3lib_div.php on line 2286

Error from 4.0RC1 install on a Clarkconnect 3 server:
Warning: file_get_contents(http://repositories.typo3.org/mirrors.xml.gz): failed to open stream: HTTP request failed! HTTP/1.1 500 ( Not implemented ) in /usr/share/typo3/typo3_src-4.0rc1/t3lib/class.t3lib_div.php on line 2286
(issue imported from #M2944)