Using SDLNet_TCP_Accept without loops

I try to connect an old Windows XP 32bit notebook with a newer Windows 7 64bit notebook via SDL_Net TCP and do some handshake send->receive->send->receive.
Both the client part (SDLNet_TCP_Open…) and the server part (SDLNet_TCP_Accept…) are only called once per frame.
When the XP notebook is the server, the connection is established fast and correct.
But vice versa i cannot connect because the SDLNet_TCP_Accept never gets a successful connection.

When i set a breakpoint behind SDLNet_TCP_Accept or do a while loop for a certain amount of time, the connection is successful.
I dont understand why the tcp connection is working when XP is the server but otherwise not.
Is it possible that SDL Net is working different on XP and Windows 7 or is there a call limit for function like SDLNet_TCP_Accept ?