SSG50 doesnt have the correct route to reach 2.2.2.2
Please check the route for 2.2.2.2 by using the command " get route ip 2.2.2.2"
Also run the debug flow basic , you will the see the drop message in the debug.
IF you still not able to find the clue of the issue , please run the following debug :
1) set ff src-ip x.x.x.x
2) set ff dst-ip x.x.x.x
3) debug flow basic
4) debug ike detail
Send the ping traffic
5) Press "ESC" to turn off the debugs
6) get db s ( paste the output)
Thanks
Atif