recv() returns BSD_ESOCK on listening socket

Hi,
MDK MDK-Plus 7.18.0 over RTX5, compiled with ARM Compiler 6 in uVision 5

I'm using BSD API in non-blocking mode.

This is a complex project (Mongoose, https://mongoose.ws) but basically the problem is with a TCP server,

Open socket, listen, clients connect, accept, configure socket, read/write, and suddenly one of the sockets starts returning BSD_ESOCK to recv() until it is closed.

I have 10 TCP sockets, 10 BSD sockets, 1 listener, and I call `listen()` with a backlog of 5. I don't have more than 3 simultaneous connections, this mostly happens with the second socket after the third connection

Error checking removed for clarity (but done), these are the calls to MDK-Plus, is there anything wrong with this ?

    fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    bind(fd, &sa, slen);
    listen(fd, 5);
    setlocaddr(fd, loc);
    unsigned long on = 1;
    ioctlsocket(fd, FIONBIO, &on);

    rc = select((int) maxfd + 1, &rset, &wset, &eset, 1 second)) < 0)
    
    do {
        clear sa
        fd = accept(fd, &sa, &sa_len);
    } while (fd == BSD_EWOULDBLOCK)
    int on = 1;
    setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, (char *) &on, sizeof(on))
    setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, (char *) &on, sizeof(on))

        n = recv(fd, buf, len, MSG_NONBLOCKING);
        n = send(fd, buf, len, MSG_NONBLOCKING);