onestep the completely empty HTML response means either your web hosting has not been configured to show a website, or there was a PHP system error. Check your webserver global/PHP error logs, and/or temporarily activate the display_errors PHP directive.
Did you already perform the Flarum installation using the web wizard, or are you stuck at the very first step after downloading the ZIP file or creating the project folder using Composer?
If you restart the installation, I recommend you perform these additional steps before using the Flarum installation wizard:
Those are easier to do prior to running the installation wizard, otherwise you'll have to modify your config file after the installation when you fix them.
If you require further help, please provide as much detail as possible about your hosting and how you installed. Is this a VPS or a managed hosting? Do you have SSH access? Did you install via Composer? What's your PHP and MySQL version? What's the webserver software? Apache, nginx, etc.
