thank you for your suggestions!
I tried with the UPnP first. I discovered, that it was off. I switched it on and it caused my wifi to drop after a few seconds with the message:
Jan 1 00:04:10 udhcpd: sending OFFER of 192.168.2.100
Jan 1 00:04:10 udhcpd: sending ACK to 192.168.2.100
Jan 1 00:04:40 syslog: IGD root device successfully registered.
Jan 1 00:04:42 syslog: Advertisements Sent. Listening for requests ...
My notebook lost wifi immediately. LAN was fine.
The other suggestion with P2P is less probable, because I tried several devices. PC, Notebook and a HTC Smartphone. All of them running barely more than a web browser.
I assume, it could have to do with the DSL modem as well. As I said, that router was working fine a year ago. Now the only change is what is connected before it. It is another DSL modem with its own settings (UPnP, Firewall, DNS, DHCP, IGMP and many more). I assume those were right too, because when connected directly to the modem, it holds the connection forever.
Is it possible, that the DSL modem somehow influences functions of devices behind it? Could it theoretically attempt to "add/delete" another route on the Edimax router via that ioctl process? To me it sounds weird, but not impossible..
I am ready to provide more usefull input to help you helping me