The Shibboleth V1 software has reached its End of Life and is no longer supported. This documentation is available for historical purposes only.
DLLPermissions
HTTP 401.3: Unauthorized Due to ACL on Resource
This error most likely means that the NTFS file permissions on isapi_shib.dll are misconfigured and IIS can't access it, and should be fixed.
For more information, see Microsoft support.