I have Grafana V8.2.1 installed in a Ubuntu VM (guess is Ubuntu, host is Windows). This error popped up when I navigate to localhost:3000 on the Ubuntu VM using Firefox. However, switching to Chromium, Grafana landing page shows up and I can navigate to my dashboard just fine.
This error shows up a few times on the forums, but none of the solutions seem related to this specific behavior. Grafana using Chromium seems fine. Grafana with Firefox seems to not work.