Error comes when add Related Contact from Account in ms crm 2015

Hi Friends,

When I add a contact (sub grid) from Account, I am getting below error.  the environment is crm 2015 online.

<ErrorCode>-2147220970</ErrorCode>
<ErrorDetails xmlns:d2p1="schemas.datacontract.org/.../System.Collections.Generic" />
<Message>System.Collections.Generic.KeyNotFoundException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #1C4C7870</Message>

But I add from some of the old record, I able to add contact in the Account form. 

Thanks,

Nagarajan