First of all: thank you for your advise. As you already indicated, my configuration was perfectly good. After digging into the details a bit using wireshark, I finally solved this issue. My WLAN router swallowed the magic packets, so they never made their way to my ASROCK mainboard. After hardwiring the sender of the magic packets, Wan-on-Lan works perfectly. As I have no idea, how to mark this thread as solved, somebody else will have to do that for me.
All the best, Florian.