After three hours of investigation I found out that one reason was a setting called Block wireless bridging I (stupidly) activated the other day on my router/access point without precisely knowing what it does :-) I thought it would do something else but according to the manual, this is what it does: it isolates all wireless clients on your network from each other. Wired clients will be seen and internet access works from Wi-Fi clients but youR17;re unable to see other wireless network clients on the same local network.