\SimpleSAML\Utils\System::resolvePath does not properly return UNC paths on Windows. E. g., for param $path being \\some-pc\some-share\some.key it returns /some-pc/some-share/some.key not being a valid path. Therefore $data = @file_get_contents($location); at \SimpleSAML\Utils\Crypto::retrieveCertOrKey fails to load the key file.