The plot thickens.
/usr/share/sandbox/userwebdav.sb lists /usr/sbin/httpd as permitted, but /usr/sbin/httpd is a symlink to /usr/sbin/httpd.i386 on my machine, which is a 32bit binary. There is also httpd.pva which is universal and a bit more than twice the size of the i386 binary.
A bit of investigation has shown that .pva binary seems to be related to parallels server which is installed on the machine too. I'll try and find out why the **** they have done this - I mean exchanging the system web server? Are they crazy?
On the bright side: after reconfiguring userwebdav.sb to allow .i386, I could get WebDAV to work correctly.
As such, I'm sorry for wasting your valuable time here. I'm guite the noob what Mac system administration is concerned, so I just didn't know about the sandbox and it's configuration.
I'll go now and make the parallels support teams live **** :-)
I think I am having the same problem as you are. My webdav was working fine for everyone but now it stopped working for one user and getting this message in /var/log/apache2/error_log "[error] (61)Connection refused: proxy: HTTP: attempt to connect to [fe80::1]:59780 (*) failed" Did you wind up fixing the problem? Would be of much help...
As I said in my response to my query: The problem was caused by parallels server. I "fixed" it by correcting the patch to apache in /usr/share/sandbox/userwebdav.sb to the one the WebDAV config was trying to execute (check /var/log/system.log).
This is at most a workaround. The correct soution is to wait for parallels to fix it, though I didn't get any response to my support request.