I already experienced several times the error : "Login failed for user '(null)'. Reason: Not associated with a trusted SQL Server connection" Could any one explain to me to what this is related?

This time there was no impact, but it could have in the future?

For information purposes only, we found what was going wrong.

The BizTalk Server is not the same as the SQL Server A user belonging to the Active Directory was used by BizTalk Server to access SQL Server

The issue in our case was related to the patch of the Active Directory that made this error.