Thank you for all your help, especially @byronyasgur for your last suggestion. It helped me fix it.
I was using markdown as the input format for the content, and the first line was HTML comment which was not followed by a blank line. Adding a blank line fixed it.
Summary: The absence of this line caused the excerpt to be null. This blnak line did exist in my development system but not on the live system.
Markdown users beware of the need of blank lines.
Sorry for the noise.
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