[Xymon] Xymon conn icon shows white, rather than green

I recently setup a Xymon server. After turn on, the web page all the "conn" item shows white, rather than green:

(I only put contents on hosts.cfg and /etc/hosts files, not going deep into any other configuration yet).

When I use fping, it shows the server is alive:

[root at island-vm etc]# fping BOOMIAPP01

BOOMIAPP01 is alive

Where is the configuration file to control it?