Using a standard L2TP config from the wiki, if you have any IP on other interfaces in the 10.255.255.0/24 subnet, the VPN will fail to route on new connections.
In my case, I had it on loopback for OSPF priority:
set interfaces loopback lo address 10.255.255.1/24
The /32 (which is what I meant to put), makes it work, but other subnet sizes don't
set interfaces loopback lo address 10.255.255.1/32