Unknown error, Precondition Failed

vdirsyncer, version 0.16.3
Python 3.5.3
Ubuntu 17.04
-vdebug output:

debug: Sending request...
debug: 412
debug: {'Cache-Control': 'private, max-age=0', 'Date': 'Mon, 01 Jan 2018 10:06:08 +0000', 'Connection': 'keep-alive', 'Expires': 'Mon, 01 Jan 2018 10:06:08 +0000', 'DAV': '1, calendar-access, calendar-schedule, calendarserver-private-events, addressbook', 'Server': 'DavMail Gateway 4.8.0.3-2488', 'Content-Length': '0'}

error: Unknown error occured for calendar_linz_ssan_to_local: Precondition Failed
error: Use `-vdebug` to see the full traceback.
debug:   File "/usr/local/lib/python3.5/dist-packages/vdirsyncer/cli/utils.py", line 74, in handle_cli_error
debug:     raise e
debug:   File "/usr/local/lib/python3.5/dist-packages/vdirsyncer/sync/__init__.py", line 153, in sync
debug:     partial_sync
debug:   File "/usr/local/lib/python3.5/dist-packages/vdirsyncer/sync/__init__.py", line 177, in run
debug:     self._run_impl(a, b)
debug:   File "/usr/local/lib/python3.5/dist-packages/vdirsyncer/sync/__init__.py", line 204, in _run_impl
debug:     href, etag = self.dest.storage.upload(self.item)
debug:   File "/usr/local/lib/python3.5/dist-packages/vdirsyncer/storage/base.py", line 15, in inner
debug:     return f(self, *args, **kwargs)
debug:   File "/usr/local/lib/python3.5/dist-packages/vdirsyncer/storage/base.py", line 15, in inner
debug:     return f(self, *args, **kwargs)
debug:   File "/usr/local/lib/python3.5/dist-packages/vdirsyncer/storage/dav.py", line 562, in upload
debug:     return self._put(href, item, None)
debug:   File "/usr/local/lib/python3.5/dist-packages/vdirsyncer/storage/dav.py", line 531, in _put
debug:     headers=headers
debug:   File "/usr/local/lib/python3.5/dist-packages/vdirsyncer/storage/dav.py", line 390, in request
debug:     return http.request(method, url, session=self._session, **more)
debug:   File "/usr/local/lib/python3.5/dist-packages/vdirsyncer/http.py", line 179, in request
debug:     raise exceptions.PreconditionFailed(r.reason)
error: 1 out of 2 tasks failed.

Start with the traceback's vdirsyncer/http.py request handler and storage/dav.py _put, upload, and request paths. Use the provided -vdebug output and the 412 Precondition Failed response as the reproduction context. Done means identifying the cause of this DavMail upload failure and defining an appropriate error or handling change.