Proxy User Account Test Works Initially And Then Fails

If I enter the username and password for a NF Server proxy user and use the Test Connection button to check the details it reports that everything is OK by displaying the Test succeeded pop-up.

I can use the Test Connection button afterwards and I still get the Test succeeded pop-up. However after a few hours if I use the Test Connection again I get the A network error occurred testing the connection and the only way I can fix it is to re-boot the Filr VM. After a reboot it works again.