Kernel Panic triggered by VPN login

I set up a PPTP server on my linode (a freshly installed Ubuntu 10.04 LTS image) and I consistently get a kernel panic when trying to VPN in from a client. This seems to happen from some clients but not from others (all using the same credentials). For those that it does happen, it happens consistently. Any ideas on how to resolve this?

The kernel panic shows this:

BUG: unable to handle kernel NULL pointer dereference at   (null)
IP: [<c0193144>] page_address+0x14/0xe0
*pdpt = 000000001eaf3007 *pde = 0000000000000000 
Oops: 0000 [#1] SMP 
last sysfs file: /sys/kernel/uevent_seqnum
Modules linked in:

Pid: 0, comm: swapper Not tainted 2.6.38.3-linode32 #1  
EIP: 0061:[<c0193144>] EFLAGS: 00010286 CPU: 0
EIP is at page_address+0x14/0xe0
EAX: 00000000 EBX: 00000000 ECX: 00000251 EDX: 00000250
ESI: 00000250 EDI: de6b7980 EBP: 20411000 ESP: df40feb4
 DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0069
Process swapper (pid: 0, ti=df40e000 task=c079df20 task.ti=c0788000)
Stack:
 deb58340 00000250 de6b7980 20411000 c049e742 00000000 000000d0 dee8a000
 00d02280 deb58be4 00000010 deb58000 deb59010 000000c0 deb58340 00000001
 deb58340 de6af8d8 c049fcff 00000000 00000001 df40ff9c dfbba380 df40ff78
Call Trace:
 [<c049e742>] ? xennet_alloc_rx_buffers+0x1c2/0x300
 [<c049fcff>] ? xennet_poll+0x4df/0xc20
 [<c04fc43a>] ? net_rx_action+0x9a/0x130
 [<c01380ec>] ? __do_softirq+0x7c/0x130
 [<c0138070>] ? __do_softirq+0x0/0x130
 <irq>[<c0137fe5>] ? irq_exit+0x65/0x70
 [<c043a03d>] ? xen_evtchn_do_upcall+0x1d/0x30
 [<c0109487>] ? xen_do_upcall+0x7/0xc
 [<c01013a7>] ? hypercall_page+0x3a7/0x1010
 [<c0105b8f>] ? xen_safe_halt+0xf/0x20
 [<c010f66f>] ? default_idle+0x2f/0x60
 [<c0107ed2>] ? cpu_idle+0x42/0x70
 [<c07ca8ac>] ? start_kernel+0x2da/0x2df
 [<c07ca410>] ? unknown_bootoption+0x0/0x190
 [<c07cdaa5>] ? xen_start_kernel+0x530/0x538
Code: 89 c2 b8 2c 1e 85 c0 e9 3f ff ff ff 0f 0b eb fe 8d b4 26 00 00 00 00 83 ec 10 89 1c 24 89 c3 89 74 24 04 89 7c 24 08 89 6c 24 0c <8b> 00 c1 e8 1e 69 c0 80 03 00 00 05 40 05 7c c0 2b 80 4c 03 00 
EIP: [<c0193144>] page_address+0x14/0xe0 SS:ESP 0069:df40feb4
CR2: 0000000000000000
---[ end trace 19ddaabd0d19ad12 ]---
Kernel panic - not syncing: Fatal exception in interrupt
Pid: 0, comm: swapper Tainted: G      D     2.6.38.3-linode32 #1
Call Trace:
 [<c063cfbf>] ? panic+0x57/0x13e
 [<c010bec6>] ? oops_end+0x96/0xa0
 [<c011e362>] ? no_context+0xc2/0x190
 [<c011e58f>] ? bad_area_nosemaphore+0xf/0x20
 [<c011e943>] ? do_page_fault+0x223/0x3e0
 [<c0184325>] ? __alloc_pages_nodemask+0xf5/0x670
 [<c011e720>] ? do_page_fault+0x0/0x3e0
 [<c063fea6>] ? error_code+0x5a/0x60
 [<c011e720>] ? do_page_fault+0x0/0x3e0
 [<c0193144>] ? page_address+0x14/0xe0
 [<c049e742>] ? xennet_alloc_rx_buffers+0x1c2/0x300
 [<c049fcff>] ? xennet_poll+0x4df/0xc20
 [<c04fc43a>] ? net_rx_action+0x9a/0x130
 [<c01380ec>] ? __do_softirq+0x7c/0x130
 [<c0138070>] ? __do_softirq+0x0/0x130
 <irq>[<c0137fe5>] ? irq_exit+0x65/0x70
 [<c043a03d>] ? xen_evtchn_do_upcall+0x1d/0x30
 [<c0109487>] ? xen_do_upcall+0x7/0xc
 [<c01013a7>] ? hypercall_page+0x3a7/0x1010
 [<c0105b8f>] ? xen_safe_halt+0xf/0x20
 [<c010f66f>] ? default_idle+0x2f/0x60
 [<c0107ed2>] ? cpu_idle+0x42/0x70
 [<c07ca8ac>] ? start_kernel+0x2da/0x2df
 [<c07ca410>] ? unknown_bootoption+0x0/0x190
 [<c07cdaa5>] ? xen_start_kernel+0x530/0x538</c07cdaa5></c07ca410></c07ca8ac></c0107ed2></c010f66f></c0105b8f></c01013a7></c0109487></c043a03d></c0137fe5></irq></c0138070></c01380ec></c04fc43a></c049fcff></c049e742></c0193144></c011e720></c063fea6></c011e720></c0184325></c011e943></c011e58f></c011e362></c010bec6></c063cfbf></c0193144></c07cdaa5></c07ca410></c07ca8ac></c0107ed2></c010f66f></c0105b8f></c01013a7></c0109487></c043a03d></c0137fe5></irq></c0138070></c01380ec></c04fc43a></c049fcff></c049e742></c0193144></c0193144>