Trunk checkout fails with serf but not neon

I cannot recreate this with the Apache repository, but with a local
1.7.5 server and current trunk client I get this:

$ svn co http://cu121.cloud.sp.collab.net/svn/desktop desktop
--username=markphip --depth=immediates
subversion/svn/checkout-cmd.c:168: (apr_err=130003)
subversion/libsvn_client/checkout.c:103: (apr_err=130003)
subversion/libsvn_client/ra.c:495: (apr_err=130003)
subversion/libsvn_client/ra.c:362: (apr_err=130003)
subversion/libsvn_ra/ra_loader.c:500: (apr_err=130003)
svn: E130003: Unable to connect to a repository at URL
'http://cu121.cloud.sp.collab.net/svn/desktop'
subversion/libsvn_ra_serf/util.c:780: (apr_err=130003)
subversion/libsvn_ra_serf/util.c:737: (apr_err=130003)
subversion/libsvn_ra_serf/util.c:1959: (apr_err=130003)
subversion/libsvn_ra_serf/util.c:1940: (apr_err=130003)
subversion/libsvn_ra_serf/util.c:2389: (apr_err=130003)
svn: E130003: The OPTIONS response contains invalid XML (200 OK)
Abort trap: 6

Using Neon it works fine.

Any ideas where to look?

On the server, you just see this:

204.11.125.142 - - [16/May/2012:16:26:37 -0700] "OPTIONS /svn/desktop
HTTP/1.1" 401 401
204.11.125.142 - markphip [16/May/2012:16:26:40 -0700] "OPTIONS
/svn/desktop HTTP/1.1" 200 191

-- 
Thanks
Mark Phippard
http://markphip.blogspot.com/