webdav failing with "Failed to query lock-null status"

I have an app that hasn't changed in a long time that uses webdav to
tranfer jpg files to my web server.

Today webdav quit working with the following message:

seiner_access_log:192.168.128.3 - webdav [28/Aug/2007:15:14:06 -0700]
"PUT /blog/Travels/images/0708/full-07240151403.jpg HTTP/1.1" 500 4485
"-" "lftp/3.5.6"

seiner_error_log:[Tue Aug 28 15:14:06 2007] [error] [client
192.168.128.3] Failed to query lock-null status for
/home/www/seiner/html/blog/Travels/images/0708/full-07240151 403.jpg

The only thing I changed is that I added a virtual server on :443, but
this is still on :80.

I've tested and I can create files in that directory, both as root and
as the www-data user.

I suspect I may have screwed something up when I added the https
protocol, but before I go off on a wild goose chase I'd like to know
what that message means.

Google turns up a few references to webdav source but alas no discussion
or explanation.

--Yan