tronix, tnx for the log of a working device!!! I suspected it but your log now definitively confirms it: my network interface eth0 is no longer initialized properly!
the line
et0: Broadcom BCM47xx 10/100 Mbps Ethernet Controller 3.90.23.0
is missing when my router initializes... this explains why a) I can not ping the router and b) I don't see any packets in wireshark.

Now I guess the only possible way to get it to work again would indeed by JTAG but since there are no visible jtag pin headers on the wl-500gp pcb I have no idea where to start ...