SYN with window size = 0

I’m trying to open a simple TCP connection to another client but my ConnectME always sends a SYN packet with a window size of zero. Using setsockopt with the options SO_RCVBUF, SO_SNDBUF, TCP_MAXSEG or TCP_SET_RCV_MSS didn’t work.

Is it possible to adjust the window size somewhere else?

Thanks, Alex