When using QSslSocket::close() behind a proxy then a device error can occur as it seems to be closing the plain socket prematurely as there seems to be more bytes to be written. If disconnectFromHost() is used instead first then it works fine. An unencrypted connection also works fine.

With the example attached, the following can occur:

on_pushButton_clicked, trying to connect 
connected 
encryptedBytesWrittenSlot sent bytes= 517 false 
encryptedBytesWrittenSlot sent bytes= 126 false 
encryptedSlot 
size sent=104857600 
bytesWrittenSlot sent bytes= 104857600 true 
encryptedBytesWrittenSlot sent bytes= 105043200 true 
QIODevice::write (QTcpSocket): device not open 
errorSlot err=-1 

To reproduce, in mainwindow.cpp change the IP address to be the one used by the proxy, and then build the application. Click on the pushbutton to see the result.