O problema que eu encontrei foi que uma seção estava faltando no arquivo application.config armazenado em C: \ Windows \ System32 \ inetsrv \ config \ Depois de adicionar a linha, o servidor FTP começou a solicitar logins.
<sectionGroup name="system.ftpServer">
<section name="log" allowDefinition="AppHostOnly" overrideModeDefault="Deny" />
<section name="serverRuntime" allowDefinition="AppHostOnly" overrideModeDefault="Deny" />
<section name="firewallSupport" allowDefinition="AppHostOnly" overrideModeDefault="Deny" />
<section name="caching" allowDefinition="AppHostOnly" overrideModeDefault="Deny" />
<section name="providerDefinitions" allowDefinition="AppHostOnly" overrideModeDefault="Deny" />
<sectionGroup name="security">
<section name="ipSecurity" overrideModeDefault="Deny" />
<section name="requestFiltering" overrideModeDefault="Deny" />
<section name="authorization" overrideModeDefault="Deny" />
<!-- the line below was missing -->
<section name="authentication" overrideModeDefault="Deny" />
</sectionGroup>