You can launch UIVI (http://www.re/be/uivi/), go to the tab "Settings" and the subtab "Proxies". The table you can edit there will write the file $HOME/.be/re/net/ProxyManager, which WebDAV-Sync will use.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
How to set a proxy for your WebDav URL?
Found a class ProxyManager, but how to configure it?
You can launch UIVI (http://www.re/be/uivi/), go to the tab "Settings" and the subtab "Proxies". The table you can edit there will write the file $HOME/.be/re/net/ProxyManager, which WebDAV-Sync will use.