Two NLB Cluster at one CAS Server

Hi Team

I have a issue, I have three CAS Server with CAS array configuration enable.

and then I have third party app  in my CAS server.

so I have two cluster nlb config in my CAS server, one is a securemail.contoso.com

the member is CAS01 and 02. this is for third party app

the other is webmail.contoso.com

this is for Exchange CAS. the member is CAS01,02,and 03

suddenly my CAS01 cant be ping and I had event id nlb 18

http://technet.microsoft.com/en-us/library/dd348820(v=ws.10).aspx

I there any misconfig with my CAS?

Thanks