The file-storage version compatible with OpenACS 5.1 is file-storage 5.0. The oacs-dav WebDAV support package is also not released yet.
The version you have is not the latest pre-release code. That bug has been fixed on the openacs-5-1 branch of CVS. The updated file-storage package and webdav support should be available with the OpenACS 5.1.1 release.