achedd this happens when the webserver doesn't return valid JSON, usually because the webserver (not Flarum) returned an HTML error page.
You can see the full HTML content by opening the browser network console under development tools and trying again.
You might also see something logged in your hosting system (not Flarum) logs.
If that doesn't help, can you describe which steps led to the error? Is this a new Flarum install, did you recently update? Are you self-hosting on VPS/shared?
