Subversion 1.7 through (squid) proxy

Following on from my earlier post, I thought I'd use my trunk Subversion
(built with ra_serf) to access some external Subversion repositories
from work.

To get access I have to use the corporate Squid proxy, and that all
works with the Subversion clients I've previously used.

However, when I try to access some Internet subversion servers using the
trunk Subversion, I encounter errors:

[Subversion 1.6.16]
$ /usr/local/subversion-1.7/bin/svn ls
http://svn.apache.org/repos/asf/subversion/trunk
svn: E020014: Unable to connect to a repository at URL
'http://svn.apache.org/repos/asf/subversion/trunk'
svn: E020014: Unspecified error message: 501 Not Implemented

[Subversion 1.6.9]
$ /usr/local/subversion-1.7/bin/svn ls
https://mrbs.svn.sourceforge.net/svnroot/mrbs/mrbs/trunk
svn: E020014: Unable to connect to a repository at URL
'https://mrbs.svn.sourceforge.net/svnroot/mrbs/mrbs/trunk'
svn: E020014: Error running context: Internal error

Accessing a local Subversion 1.6 server over HTTP (i.e. without a proxy)
works fine...

Has "HTTPv2" been tested through a proxy at all?

Note: I have ensured Squid is compliled to support a number of
additional HTTP methods to support Subversion:

# grep OPTIONS /etc/squid/squid.conf
extension_methods REPORT MERGE MKACTIVITY CHECKOUT PROPFIND
PROXY-AUTHORIZATION OPTIONS

The same trunk Subversion built at home, not going through a proxy works
fine...

John.

-- 
John Beranek                         To generalise is to be an idiot.
http://redux.org.uk/                                 -- William Blake