I haven't been able to get L2TP working on my USG 50 either. I followed the example in section 4.6.1 of the User's Guide
as well as this iOS doc
, and I can't get Windows or iOS clients to connect successfully.
For Windows XP/7, I get "[SA] : Tunnel [Default_L2TP_VPN_Connection] compatible IKE rule mismatch" despite configuring the Phase 2 stuff as per the examples and for iOS I get "SPI:0x0 SEQ:0x0 No rule found, Dropping packet" followed up by a "User L2TP-test has been denied from L2TP service.(Incorrect Username or Password)" despite the fact that this account/password works fine with my existing IPSEC connection.