Short update here:
I tried to fixed the Google Scholar fetcher and - good news - it is working again in the development builds available at https://builds.jabref.org/master.
However, the number of downloadable results is now limited to only the first 10 Google hits. Trying to download more than those 10 hits would instantly block the usage of JabRef with your IP address.
We are working on a complete rewrite of the fetcher which will hopefully overcome this limitation.