I managed to turn on WPA2.
it took me some time since it only worked after I pressed the reset button on the back of the router
I've completely worked my way trough the web-admin again to reconfigure everything, and now it's working stable.
furthermore, I can switch between encryption modes now
Also the ethernet bridge between my wl-500w and my rt-n16 is better now with wpa2, I can get a troughput of ~6MB/s sometimes
but usually around 2MB/s