Ahh... got it.
The details I failed to notice was:
m_listen() returns a server socket thing, which should not be used for communication or closed. There is no need to open or close new sockets (or start something which results in this) on connect in the m_listen-callback function.
Several (all?) of the TCP server examples here does not seem to pay enough attention to this.
Now, the next thing:
At some point (e.g 3 connections), I would like to refuse additional connect rather than failing or crashing. Has anyone succeeded in this without closing the already established connections?
(I seem to be replying to myself only - is anybody alive in this forum?)