vasc check the Flarum logs at <flarum>/storage/logs and your webserver logs for details on the 500 error.
For the webroot, either: it's a VPS, and you can edit the virtual host / site config and change DocumentRoot (apache) or root (nginx). If it's a managed service (shared hosting), there should be a setting somewhere to customize the domain root (by default public_html). If none of this is possible, see https://flarum.org/docs/install.html#customizing-paths . Please note that there are a few changes in the latest Flarum version and there's now a site.php simplifying where to edit some of the paths.