OprekMania that's definitely not an error that was handled properly. Maybe check the browser network tab (in the development tools) to see what happens. Maybe the request to start the installation was blocked by a proxy or something like that.
Before installing you should make sure to change the DocumentRoot setting of your website to point to the public folder, or alternatively follow https://flarum.org/docs/install.html#customizing-paths so that /public doesn't end up at the end of your forum url.
