Yann,
UDP traffic does not have a "SYN" so it will always say "no syn". Don't worry about it.
1. You should do "show datapath session table <outer ip address of ap>" - done
2. You should do "show crypto ipsec sa" - done
3. You should do "show datapath session table <inner ip address of rap>" <------This is what I am interested in.
4. Turn on security debugging:
config t
logging level debugging security subcat ike
logging level debugging security process aaa
logging level debugging security process authmgr
logging level debugging security subcat l2tp
logging level debugging security subcat vpn
5. Type "show log security 50" when the RAP is coming up to see if you see anything.