OK - Another thing to try. If you haven't already done so, install the Assetto Corsa Competizione Dedicated Server from Steam. Check where steam installs it. Go to view=->Settings on Steam
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Go to Downloads->Steam Library Folders and check the default folder.
Now go to the folder <Steam Folder>\steamapps\common\Assetto Corsa Competizione Dedicated Server\sdk\broadcasting\Testclient
Start Kunos' test broadcasting client - ksBroadcastingTestClient.exe. Now start an ACC session (the Drive menu is sufficient). In the test client, press connect
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If ACC's broadcast interface is working, then you'll see sall the cars in the sesssion
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If it works, then it is a problem with my app. If not can you zip up the folder C:\Users\<your user name>\AppData\Local\AC2\Saved\Logs - we'll need to look at ACC logs to see if there is an issue there