Some healthcheck.htm requests do not respond (dropped connection)

Hi,

Several (most) of the services on our Exchange server don't return a valid response when querying their "health check" URL.

These are OK:

 - /ecp/healthcheck.htm (200 OK)
 - /owa/healthcheck.htm (200 OK)
 - /powershell/healthcheck.htm (200 OK)

But these fail with an instantly dropped connection:

 - /autodiscover/healthcheck.htm
 - /ews/healthcheck.htm
 - /mapi/healthcheck.htm
 - /Microsoft-Server-ActiveSync/healthcheck.htm
 - /oab/healthcheck.htm
 - /rpc/healthcheck.htm

There are no errors or warnings noted in the event logs, and all services work normally for all users, but I don't see the failed requests in IIS' logs.

What can be causing this?

Thanks!