You should have marked "AsyncPostBackTimeout" in RED-BOLD
Back to your question, As you have precisely figured out , it indeed does not look like a problem with the server. Presuming, that both the test machines (clients) are on the same network, here is what i need you to check out.
1. Have you used the same version of browser on both the test machines? Could that be a clue?
2. Is there a proxy set in one of the machine browsers? Tools- Internet Options - Connections - Lan Settings (IE)?
3. did you try clearing your browser cache - Tools - Internet Options - Delete.,
4. Add an AsyncPostBackError handler and try to see if you can get any additional info. .Net messages are better known to hide the actual error in the innermost exceptions.
The script would look like,
protected void ScriptManager1_AsyncPostBackError(object sender, AsyncPostBackErrorEventArgs e)
{
//e.Exception.Data["ExtraInfo"]
}