I have observed a few times while deploying software into IIS that every now and again the related Application Pools fail to restart and in the Event Log we see an error like the following:

The identity of application pool, 'AppPoolName' is invalid. If it remains invalid when the first request for the application pool is processed, the application pool will be disabled.

This does not happen frequently but when it does the only solution is to re-apply the Identity password in the IIS Manager Window. As soon as I re-apply and then restart the application pool the web sites come back up.

Facts:

• The account is a service account whose password never expires.
• The account is local to the IIS host.
• The account password is never changed.
• This is IIS 6 running on Windows Server 2003
• Deployment of the software is via MSI and involves several IIS Resets.
• The software is created in house and does not do anything fancy to IIS.

Any ideas how the identity information might become corrupt?

Edit: Clarification

To be clear - this user account and password combination work absolutely fine and usually works fine as the Identity of the Application Pool. It is only when we deploy updates of our software into an existing IIS application that it stops working.

• Our password has not changed
• Our deployment does not change the password or reconfigure the application pools.
• This does not happen every time, 1/20 times perhaps.
• If we re-enter the password into IIS and restart the App Pools everything works.