Found it. I had my nas configured with a static ip address. I filled in ip address, netmask and gateway. I did not fill in dns and expected my gateway to take care of that. This worked with firmware 1.05. Seems that 1.06 doesn't accept these setting. I tried to fill in the dns fields but it doesn't stick, they're blank after I press save.
So now I have the nas configured as dhcp cleint. I let my router take care of assigning a static ip adress. And now the nas can find the nameservers. And BT works.