Offline Client Despite the Fact It is Online

Dear all,

I have two ubuntu clients in the same network, where one doesn't have problem, but the other is always recognized offline by the server.

In the deemed offline machine, I restart and enable daemon without error. And ping to other sites without problem. However, the server always sees it as offline. How can I troubleshoot this? I'm using  13.1.3026  (DEB).

regards