kejunchen make sure you login into Flarum before accessing the admin panel, because it will not redirect you to the login page automatically.
If you still have issues, what does the page look like? The Flarum-generated error page will have a custom font and text centered. The Apache or nginx error page will likely use the default serif font with no CSS.
If the error is generated by the webserver, take a look in any firewall, proxy or server logs for a reason.
If the error page is generated by Flarum, then it could be an extension causing an issue.