I probably found a problem in close function of FTPClientSession, here is the function:

void FTPClientSession::close()
{
	logout();
	_pControlSocket->close();
	delete _pControlSocket;
	_pControlSocket = 0;
	_serverReady = false;
}

it's called in destructor of FTPClientSession as:

	try
	{
		close();
	}
	catch (...) 
	{
	}

So it's safe: the problem here is in the logout function inside , because it does something like

		try { endTransfer(); }
		catch (...) { }
		std::string response;
		sendCommand("QUIT", response);
		_isLoggedIn = false;

If, as in my environment, the sendCommand throw exception because socket is broken or whatever, you end up in FTPClientSession::close() exit too early without do the close of the FTP control socket.
Probably in close() function logout should be under try catch