I have the same problem with one of my friends. Text chat always failed with "Transmission not sent...", i have verbose logging on and spotted this in the log:
{17:11:37} IJoinSession:Talk: 0x000000024bd21eb6: 109.47.70.83:64335 x 265 Name Unknown
{17:12:07} FailToJoinSession1: 0x000000024bd21eb6: at 30.0s : 109.47.109.112:55882 x 4 # Name Unknown
{17:12:08} Machine 109.47.109.112:55882 x 3 # Name Unknown disconnected: Unable to establish connection with SYNC
{17:17:49} machines=2&numturnlinks=0&backlogtotal=0&backlogmax=0&avgsrtt=183&loss=0.000&&jit=0.270&act1=39.040&act2=1.110
{17:18:19} FailToJoinSession1: 0x000000024bd21eb6: at 30.0s : 109.47.109.112:55882 x 4 # Name Unknown
{17:18:19} Machine 109.47.109.112:55882 x 3 # Name Unknown disconnected: Unable to establish connection with SYNC
{17:27:49} machines=2&numturnlinks=0&backlogtotal=0&backlogmax=0&avgsrtt=183&loss=0.000&&jit=0.149&act1=41.269&act2=0.989
{17:37:49} machines=2&numturnlinks=0&backlogtotal=0&backlogmax=0&avgsrtt=240&loss=0.000&&jit=0.104&act1=40.707&act2=1.006
{17:41:16} FailToJoinSession1: 0x000000024bd21eb6: at 30.1s : 109.47.109.112:55882 x 4 # Name Unknown
{17:41:16} Machine 109.47.109.112:55882 x 3 # Name Unknown disconnected: Unable to establish connection with SYNC
{17:45:26} FailToJoinSession1: 0x000000024bd21eb6: at 30.0s : 109.47.109.112:1027 x 4 # Name Unknown
{17:45:26} Machine 109.47.109.112:1027 x 3 # Name Unknown disconnected: Unable to establish connection with SYNC
{17:47:49} machines=2&numturnlinks=0&backlogtotal=0&backlogmax=0&avgsrtt=181&loss=0.000&&jit=0.080&act1=44.315&act2=0.917
{17:48:32} FailToJoinSession1: 0x000000024bd21eb6: at 30.0s : 109.47.109.112:1027 x 4 # Name Unknown
{17:48:32} Machine 109.47.109.112:1027 x 3 # Name Unknown disconnected: Unable to establish connection with SYNC
{17:50:21} FailToJoinSession1: 0x000000024bd21eb6: at 30.0s : 109.47.109.112:1027 x 4 # Name Unknown
{17:50:21} Machine 109.47.109.112:1027 x 3 # Name Unknown disconnected: Unable to establish connection with SYNC
{17:50:59} Machine 109.47.109.112:1027 x 3 Name Unknown disconnected: Unable to establish connection with SYNC
{17:57:49} machines=2&numturnlinks=0&backlogtotal=0&backlogmax=0&avgsrtt=189&loss=0.000&&jit=0.065&act1=51.138
Note: my friends IP is the one with .112 on the end (don't bother checking the ip, we change it every time while re-dial DSL)
And with other commanders in vicinity @ SC, text chat worked. And there's no SYNC error. Whatever that means.
Need FD to clarify what "Unable to establish connection with SYNC" means. We have disabled all firewall, use DMZ to allow all ports to be redirected to the sole computer. Tried with uPnP on/off, port forwarding. Nothing helps..
I'm going to try to let him use my network since he's here now and going to post an update how it went here.