L2TP problems

My provider has changed protocol from PPP to L2TP. And now I can’t use Comodo FW because of Network Monitor blocks my outgoing L2TP packets.
I like this FW, so should I ask you to help me solve this problem?

OS: WinXPpro+SP2+all critical patches+avast!.

===cut from log===
Severity :High
Reporter :Network Monitor
Description: Blocked by Protocol Analysis (Fake or Malformed UDP Packet)
Direction: UDP Outgoing
Source: myIP:1701
Destination: serverIP:1701
Reason: UDP packet length and the size on the wire(705 bytes) do not match
===cut from log===

PS: Everithing goes perfect if I switch Security Level to “Allow All”.