[Owncloud] How to debug "HTTPError: 400 Client Error" when client sends new vCard to Owncloud?

[Chris Green]

I am trying the command line CardDAV client pycardsyncer (part of the
pycarddav package) with Owncloud.

It reads vCards from Owncloud with no problems and creates its own local
database of address records for use by the other programs in pycarddav.

However when I add a new records to pycarddav's database and then try to
synchronize with OwnCloud I get the HTTPError: 400 Client Error.

Looking in the apache logs there are no errors reported and there's
nothing obvious wrong in the access.log.

I have also got Thunderbird set up with the SoGo CardDav addon and that
can successfully send new vCards to Owncloud.  The logging in apache's
access log looks identical (except for user agent) for both Thunderbird
and pycarddav.

Thus I *think* the problem may be in the actual content of the vCard
that pycarddav is sending, the trouble is that I can't get any error or
debug logging out of Owncloud that would explain why it's returning an
error.  How can I find where it's deciding something is wrong and
sending the error?

-- 
Chris Green