Hello,
The session table information looks incorrect. With a destination NAT, session should be something like:
In: 151.**.**.**/500 --> 10.10.10.2/500;udp, If: ge-0/0/5.0, Pkts: 2, Bytes: 692
In: 20.20.20.1/4500 --> 151.**.**.**/4500 ;udp, If: (interface that goes to 20.20.20.1 server>, Pkts: 18, Bytes: 3913
Can you cross check the session table or provide new session entry when security policy has junos-l2tp enabled?
Also which ALGs do you have enabled on SRX?
Regards,
Rushi