Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
How To use iChat with a NAT router or a firewall on Mac OS X 10.5
While chances are slim that you will need help with chatting in iChat through firewalls and NAT routers on Mac OS X 10.5, we mention the possible fixes that may hinder your chatting experience.
In general the default configuration of Firewalls or Routers permit chatting through iChat to pass through without any need to change the settings.
Note: Keep in mind that if you face issues with iChat on Mac OS X 10.5, not all suggestions made here will stay valid.
Working With Firewalls and Routers
A network connection usually involves firewall, routers or a combination of both.
With Mac you need not install any Firewalls as they are built in. If your Mac is hooked on to the internet directly such as through a broadband without any routers, Mac allows iChat traffic trough to function correctly.
On the other hand, if your Mac is connected to the net through a wireless base station or a router such as Airport Extreme, you will have to ensure that the settings allow iChat exchange. To do so, you may read more about configuring these devices by consulting the documentation booklets provided with the devices.
However, if you use a combination of the two, routers and firewalls, chances are high that you will need to configure both of them to allow iChat perform video and audio chatting properly.
iChat Port Requirements For the Newer Versions
iChat on Mac OS X 10.5 uses UDP port 16402 to let the traffic through for video-audio chatting. This is a default setting with iChat and Mac OS X 10.5. Just in case, 10.5 finds that the port 16402 is not available, it tries to look for other ports until the next ten ports. The moment it finds an available port, it uses it to let the traffic through. Therefore ensure that you include the 16393-16402 ports on your routers and firewalls. Just in case, none of these ports are available, Mac OS X 10.5 makes a last effort to connect by choosing a random port and trying that.
The new iChat on Mac OS X 10.5 requires only one port to function properly. Unlike the older versions, it doesn’t require two ports to be open simultaneously. Therefore the moment Mac finds one available port, all the Av traffic and signaling is transmitted through it.
So, if you have firewall and routers that let you configure them, ensure to include the range 16393-16402 for ports. You also need to ensure that Port 5678 can be used for connecting to the external host for SNATMAP service, which helps iChat clients to exchange data with one another, even when they are behind a firewall.
iChat Port Requirements For Older Versions
Whenever a Mac OS X 10.4 and Mac OS X 10.5 connect through iChat for a video or an audio chat, 10.5 Mac uses a single port, which in most cases is 16402, on the other hand, a10.4 Mac can use a grouping of five different ports.