Google Scholar is a freely accessible database that indexes the full text or metadata of scholarly literature across an array of publishing formats and disciplines. Google Scholar index includes most peer-reviewed online academic journals and books, conference papers, theses and dissertations, preprints, abstracts, technical reports, and other scholarly literature, including court opinions and patents (Wikipedia).
Google scholar can block "automated" crawls which generate too much traffic in a short time. To unblock your IP, doing a Google scholar search in your browser might help. You will be asked to show that you are not a robot (a CAPTCHA challenge). If no CAPTCHA appears, or JabRef is still blocked after performing a search in the browser, you can also change your IP address manually or wait for some hours to get unblocked again.
Thus, the Google Scholar fetcher is not the best way to obtain lots of entries at the same time. The JabRef browser extension might be an alternative to download the bibliographic data directly from the browser.