BizTalk Server 2006 - Host instance wouldn't start because of bad configuration

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You may have this kind of event when a host instance starts

Event Type: Error
Event Source: BizTalk Server 2006
Event Category: BizTalk Server 2006
Event ID: 5410
Date: ...
Time: ...
User: N/A
Computer: ...
Description:
Failed while executing a Windows service request.
Service request: Start

BizTalk host name: BizTalkServerApplication
Windows service name: BTSSvc$BizTalkServerApplication

Additional error information:
Error code: c0c0153a
Error source: BizTalk Server 2006
Error description: A BizTalk subservice has failed while executing a service request.
Subservice: Tracking
Service request: Start

Additional error information:
Error code: 80131534
Error source: Microsoft.BizTalk.Bam.EventBus
Error description: The type initializer for 'System.Data.SqlClient.SqlConnection' threw an exception.

 

This happens when config file is changed. The host instance config file is typically at C:\Program Files\Microsoft BizTalk Server 2006\btsntsvc.exe.config

Note that this config file can have a problem in encoding or because of a bad character. Try copying it as an xml file (eg.: copy of btsntsvc.exe.config.xml) and opening it with IE.