same problem on my kirkwood device. 'Fixed' by blacklisting mv_cesa module in /etc/modprobe.d/blacklist
Seems to be a bug in mv_cesa with ipsec in type=transport mode?! I hope this gets resolved, as hw crypto acceleration is really nice on kirkwood for dmcrypt/cryptsetup.
by the way, xl2tpd performs really abysmally on my arm boxes! (haven't tried on x86). Is this a common problem for xl2tpd? openl2tpd has much better performance, but is not the end-all as it has its own, different, issues.