WebDAV Publishing is an IIS role/feature that can cause certain operations within the Smart API Manager web client to fail. This feature interferes with the HTTP PUT and DELETE verbs. So, deleting a Group or User, for example, can fail.
Uninstalling the WebDAV Publishing role fixes these problems.
Some users setting up an IIS web server may turn on all features, which is not advised as it can cause unexpected issues. If the web server actually requires the WebDAV Publishing role, search the web for other solutions.