The "Malformed gateway or interface not found" message means that routing daemon is not aware of pppoes72 interface which is strange because you see it via show pppoe-server.
Lets compare list of interfaces from different sources:
- List of linux interfaces - sudo ifconfig
- List of routing daemon interfaces - show interfaces
- List of pppd interfaces - show pppoe-server
- Connected route of missing interface from routing daemon - show ip route y.z.12.171
- Connected route of missing intrerface from linux - ip route | grep y.z.12.171