ppp using rp-pppoe plugin only allows limited interface names - "ethXXX", "nasXXX", "tapXXX" or "nic-XXXX". With is very silly really, as many wireless devices have default names such as wifi0, wlan0, ath0, ra0, etc - which is the main reason for this patch. What makes this sillyness even worse is that you can change the name of these interfaces to wickywoo0 if you really wanted too.
Created attachment 64412 [details, diff] rp-pppoe remove interface name check
fixed in ppp-2.4.3-r7 and ppp-2.4.2-r13