Biztalk got an error “A transport-level error has occurred when receiving results from the server”

Our Biztalk (2006R2) faced an error as below:

Event Type: Error Event Source: ENTSSO Event Category: Enterprise Single Sign-On Event ID: 10514 Date: 2/14/2014 Time: 5:02:27 PM User: N/A Computer: NINI-SSPP03 Description: An error occurred while attempting to access the SSO database. Function: GetGlobalInfo File: infocache.cpp:1349 A transport-level error has occurred when receiving results from the server. (provider: TCP Provider, error: 0 – The specified network name is no longer available.). SQL Error code: 0x00000040 Error code: 0xC0002A21, An error occurred while attempting to access the SSO database.

But we cannot find any error in SQL Server event log and SQL Server log.

Is there any idea on the possible cause of this error?

Thanks.