Error 404 – File Not found (.html automatically added to url)

Help Fellow Members!

My site is: https://aboutinternetetc.com

I did this:
Recently and I am not sure this is the cause of my current issue: Changed Permalinks settings inside WordPress SEO to “Custom Structure” >> /%postname%/ from > “Day and Name”.

I expected the plugin to do this:
Simply begin posting articles accordingly

Instead it did this:
Recently I just started getting the following error: Error 404 – File Not Found.

The peculiar thing is that this error only occurs whenever I try to access my post(article) from a serp after a Google search on the following search string “Internet security with Microsoft Security Essentials” (I use the quotes in the search string).

On the other hand, if I simply navigate to my site and click on the article title, I am properly taken to the article without any problem…no errors.

This only seems to happen whenever I try to access the articles from a Google serp containing the url link.

Also, please Note: “.html” is always found at the end of my url when I finally land on the page with the “Error 404 – File Not Found”. This is not the case when I access the article-post via my website. There is no .html at the end of the page url and there is no error 404 – file not found.

Has anyone out there ever experienced this same problem or symptoms? If so, what was the fix. Help!

Thank you,
-Dan