I have just discovered that the error is due to an invalid format of .aws\credentials file content.
It can be solved by editing the file and placing there proper credentials, following the format shown here:
Anyway, to get a useful tip which addressed me to the resolution, I had to uninstall AWSCLI from its MSI installer package and then reinstall it using pip:
pip install awscli --upgrade --user
After some more troubleshoot of PATHs, I ran aws again and I got a more meaningful error message, which addressed me to the resolution above.
Thank you for your reply really helped me!
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