haproxy NOSRV issue with redirected port

There is no default backend to use and rest_cluster is used only when host_rest ACL is valid.
So any request with a HOST header not matching "mypublicserver.myhost.com" will not be routed to any backend and this results in a 503 error.
So you can either add a default_backend directive or drop/edit the ACL.

EDIT: if the ACL is not matching this is because it is missing the port part: 8080
Try with:

 acl host_rest hdr(host) -i mypublicserver.myhost.com:8080