Cannot run app pool as service account

I had a Python Flask website that ran fine when I ran the app pool as my own account. When I tried changing this to a service account (which should have permissions), I get the following error

HTTP Error 500.0 - Internal Server Error

The page cannot be displayed because an internal server error has occurred.

Most likely causes: •IIS received the request; however, an internal error occurred during the processing of the request. The root cause of this error depends on which module handles the request and what was happening in the worker process when this error occurred. •IIS was not able to access the web.config file for the Web site or application. This can occur if the NTFS permissions are set incorrectly. •IIS was not able to process configuration for the Web site or application. •The authenticated user does not have permission to use this DLL. •The request is mapped to a managed handler but the .NET Extensibility Feature is not installed.

Detailed Error Information: Module FastCgiModule

Notification ExecuteRequestHandler

Handler Python Flask

Error Code 0x80070542

Requested URL http://qatools:80/

Physical Path D:\PythonWebService\QATools

Logon Method Anonymous

Logon User Anonymous

If I change it back to run as my own account, it works. Any thoughts?

FYI after some tinkering, I changed the app pool to run with Managed Pipeline mode of "Classic" instead of "Integrated" and it worked.