Thanks for your report.
I'm sending you an email with a development version of WinSCP for testing to address you have used to register on this forum.

Hello,
After some investigation I've found that WinSCP is the best
WebDAV client (even unreleased yet) for Windows 7. But there is
an annoying issue with it:
WinSCP version 5.2.0 build 2895 (from nightly builds)
Exporer-like interface view
When drag-n-dropping a folder with files in it to remote webdav directory,
we get "PUT 409 error". An empty folder's being uploaded OK.
Other clients (we tried davfs on Linux and BitKinex on Windows) perform this
without errors.
We have a demo version of our software implementing WebDAV server here,
it can be used for testing:

hostname: demo.arta.kz
port: 80
user: 111111
password: 111111
remote directory (mandatory): /webdav/repository/default/company_root

The demo is being erased and re-uploaded every night, so feel free to make
any experiments there.