If after upgrading your self-hosted helpdesk from "9.x" to "10.x" all API calls start returning "404 - File or Directory not found" try this:
Open the file "
C:\Windows\System32\inetsrv\Config\applicationHost.config
" and check if there's a section that configures access to "/api" location. Something like:
<location path="Default Web Site/helpdesk/api">
<system.webServer>
<security>
<authentication>
<anonymousAuthentication enabled="true" />
</authentication>
</security>
</system.webServer>
</location>
Simply remove that section and it should work. It was required for older versions, but not the one based on ".NET Core" (aka NET 5 and later)