tbscope is right...

When you connect proxyServer::newConnection and WebProxy::go(), they are on the same thread, so it is a direct connection. When this happens, Qt doesn't fill up the sender() value... so, when you do it in your wpt->start(), which calls run(), you are using a null value.
As a try, you can replace
Qt Code: 
  1. QTcpServer *proxyServer = qobject_cast<QTcpServer*>(sender());
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by
Qt Code: 
  1. QTcpServer *proxyServer = qobject_cast<QTcpServer*>(parent());
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BTW, I think you will need at least one mutex to protect the following
Qt Code: 
  1. QTcpServer *proxyServer = qobject_cast<QTcpServer*>(sender()); //this you should review
  2. QTcpSocket *socket = proxyServer->nextPendingConnection();
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to avoid lost connections.
Try making this a piece of WebProxy implementation, instead of doing this in the thread start...

Or better: catch the nextPendigConnection, and pass it to constructor of the thread object (using socket descriptor, not sure if you can use a QTcpSocket across different threads)... and then use the QAbstractSocket::setSocketDescriptor at thread startup...

hth.