This issue occurs if the both the following conditions are true:
- You try to access the remotely stored content as a user from a trusted domain that is located in a different forest.
- There is an external cross-forest trust configured between the two domains.
External trusts only support Integrated Windows authentication (formerly called NTLM) for the user access. Therefore, typical Server Message Block (SMB) access to the target share that you reference in the WebDav directory is unsuccessful.
In this scenario, Microsoft Internet Information Services (IIS) pass-through authentication is unsuccessful even if protocol transition is enabled for IIS. Protocol transition for Integrated Windows authentication only works in the same forest. This is because a transitive Kerberos trust is available in the forest. A transitive Kerberos trust enables IIS to issue a Kerberos ticket on behalf of the requesting user (delegation). A transitive Kerberos trust is not available over an external cross-forest trust because the external Kerberos realm is unknown in your forest.