vpn problem cisco & watchguard

> I have a problem with a cisco router ed Watchguard firewall.
> Sometime the tunnel ipsec dropped.
> the logs on the router is that:
> %CRYPTO-4-IKMP_PKT_OVERFLOW : ISAKMP message from [IP_address] larger ([dec]) than the UDP packet length ([dec]) 
> Explanation ISAKMP messages are carried in UDP packets and have their own message length field. The message length field of this message was greater than the length of the UDP packet. This situation could indicate a denial-of-service attack.
> Recommended Action Contact the remote peer and the administrator of the remote peer.
> 
> the remote watchguard 700/III release 7.0 sp1 seem not have a problem.
> the cisco si that:
> System image file is "flash:c837-k9o3y6-mz.122-13.ZH2.bin"
> CISCO C837 (MPC857DSL) processor (revision 0x400) with 29492K/3276K bytes of memory.
> 
>  Please let me
> know this as soon as possible.
> 
> Thanks in advance.
> 
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