usg 1000 vpn 16015 error

@kriszty As I know all of client traffic will been forwarded into tunnel after L2TP tunnel is established.

There are the other ways can separate traffic:
(1) IKEv2....it is only works on NextGen model.(e.g. USG110)
(2) SSL VPN with full tunnel mode. (client needs installing SecuExtender)

These ways, client's internet traffic will use itself but not ZyWALL.