I get the impression the webpage is not properly html formatted. It appears to be just php dumping text. If you open the url in a browser and check the source you'll see what I mean. Since you're using the HTML Table reader I would expect that it needs a </table> and a </html> tag.
Funny enough it only happened the first time I tried it. I added an inspector after the StringReplacer and since then the workspace is properly stopping. But the page source is still bad.
For what it's worth, I was unable to reproduce the behavior using FME 2017.1 and 2018.0 beta.
Also I agree with @redgeographics, the html is invalid. Just look at the page source for https://www.oil-price.net/widgets/brent_text/gen.php?lang=en