Did you just update that server to 3.0? In that case, check if the
update changed the port of the wsus webserver instance to 8530. Judging
from the URLs in your log, your clients are configured to use port80,
and at least in my case, the update chnaged the port from 80 to 8530
without notice, resulting in exactly your error.
Resolution: If you don't have anything else runnign there (which you
shouldn't), either stop or chnage the port of the default webserver
instance to something else than 80, and than chnage the port of the WSUS
webserver instance back to 80. Then, in the WSUS MMC, create a new
server entry pointing to port 80, and delete the original one (which
points to port 8530).