Can you create manually a small folder and files and see if it can't create folders at all? Might be interesting to check the owncloud and webserver logfiles at the moment when Robocopy runs into this error. Which way are you syncing from owncloud or to owncloud (idea: some file and folder names are not allowed in Windows)?
To exclude owncloud, you could perhaps try a different webdav server or a different owncloud setup (different version). There is also an external tool that allows you to test a webdav storage: