I've been using Flubaroo for 5 years and have never had this problem:
When I set up new Google Forms I routinely set up autograding so that students receive grade EMails. So to this, of course, I click "Enable Autograde" in the Google Sheet. Next I do Step 1 (no problem). Then Step 2 (choose a key... no problem). Step 3 is called "Share Grades". I always uncheck "Include list of questions and scores" and then click "Continue".
For 5 years now, this last step has completed in several seconds. Starting this weekend, however, this last step (Share Grades) got stuck. I see a progress circle spinning near the lower right, but it spins forever. When students submit, they do not receive their grade EMails.