I turned the powermanagement off, and since then the wifi has been working fine. So the problem seems to be with the powermanagement. I would like to note that before todays update the wifi was working flawlessly. (I don't know if the update turned on the powermanagement of the wifi automatically. I have a feeling i had it on before too.) My WLAN router is Buffalo nfiniti WHR-G300N Ver.1.60 (R1.49/B1.09)