Team Foundation Server stopped working all of a sudden

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Hi there,

Have had Team Foundation Server working for months now and all of a sudden
after a restart of IIS it has been failing with errors:

The identity of application pool 'TFS AppPool' is invalid, so the World Wide
Web Publishing Service can not create a worker process to serve the
application pool. Therefore, the application pool has been disabled.

For more information, see Help and Support Center at
http://go.microsoft.com/fwlink/events.asp.


and


A failure was encountered while launching the process serving application
pool 'TFS AppPool'. The application pool has been disabled.

For more information, see Help and Support Center at
http://go.microsoft.com/fwlink/events.asp.
The identity is a member of IIS_WPG group and also has Log on Locally rights.
How can I troubleshoot this problem? I even created a new identity which
failed as well. Could this error be related to something else?