s2sVPN - vpnd crashing every 10 seconds

I am running 10.6.8 server on a late 2008 quad core xserve. AFP, VPN (IPSEC over L2TP using shared secret), DNS and OD enabled services.

I have set up a site to site VPN using the s2svpnadmin utility between a Leopard Server 10.5.8, and my primary 10.6.8 server. On the 10.6 server when I start the vpn service I get the following error message in my logs every 10 seconds:

Server 'testserver' starting...

Jul 15 11:33:43 myhost vpnd[4966]: Listening for connections...

Jul 15 11:33:43 myhost testserver[4966]: 2011-07-15 11:33:43 BST Listening for connections...

Jul 15 11:33:43 myhost com.apple.launchd[1] (testserver[4966]): Job appears to have crashed: Segmentation fault

Jul 15 11:33:43 myhost com.apple.launchd[1] (testserver): Throttling respawn: Will start in 10 seconds

Jul 15 11:33:43 myhost com.apple.ReportCrash.Root[4942]: 2011-07-15 11:33:43.783 ReportCrash[4942:2583] Saved crash report for vpnd[4966] version ??? (???) to /Library/Logs/DiagnosticReports/vpnd_2011-07-15-113343_localhost.crash

The crash report contains the following:

Process: vpnd [5006]

Path: /usr/sbin/vpnd

Identifier: vpnd

Version: ??? (???)

Code Type: X86-64 (Native)

Parent Process: launchd [1]

Date/Time: 2011-07-15 11:34:44.130 +0100

OS Version: Mac OS X Server 10.6.8 (10K540)

Report Version: 6

Exception Type: EXC_BAD_ACCESS (SIGSEGV)

Exception Codes: KERN_INVALID_ADDRESS at 0x0000000000000000

Crashed Thread: 0 Dispatch queue: com.apple.main-thread

Thread 0 Crashed: Dispatch queue: com.apple.main-thread

0 ??? 000000000000000000 0 + 0

1 vpnd 0x00000001000027c3 main + 1707

2 vpnd 0x000000010000167c start + 52

If I turn the VPN service off again through server admin, the error persists. If I delete the "testserver" vpn configuration through s2svpnadmin utility I get an error saying could not load preferences file until I reboot the machine. The s2s vpn does not work no matter what I do. Has anyone seen this before? What does the segmentation fault indicate, and why would vpnd be crashes every 10 seconds?

Many thanks in advance