An IIS website may display “HTTP 503 – Service Unavailable” error when the application pools don’t have correct configuration or permissions which causes them to unexpectedly stop. Event ID 5059 clearly shows the reason behind the 503 error: “Application pool has been disabled”.
You may also see 5021 (The identity of application pool %1 is invalid) and 5057 (Application pool %1 has been disabled) in Event Viewer. Here are the detailed error messages:
Event ID 5021
The identity of application pool %1 is invalid. The user name or password that is specified for the identity may be incorrect, or the user may not have batch logon rights.
Event ID 5057
Application pool %1 has been disabled. Windows Process Activation Service (WAS) did not create a worker process to serve the application pool because the application pool identity is invalid.
Event ID 5059
Application pool %1 has been disabled. Windows Process Activation Service (WAS) encountered a failure when it started a worker process to serve the application pool.
HTTP 503 status code and 5059 event ID may indicate issues related to account permissions or IIS configuration. Here is a list of things to try. You may need to restart IIS or machine after each change.
IISCngProvider) in applicationHost.config
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