What do you see when you load the homepage ? Is the navigation and sidebar present ? Does the same error message appear when you change the ordering of the discussions (even if there are probably no discussions) ?
Is the error really the same when you try accessing the admin panel ? Does the header of the admin panel show up ? Or is it a full page error ?
If you enable debug mode in config.php then try again and check the browser network tab, you should be able to see the full error message returned by the backend, like an actual ModelNotFound, or RouteNotFound error. If you try accessing the /admin page you won't even need to go to the browser network tab, you will see the error message right on the page itself.
Without any more details, my guess is a RouteNotFound exception, probably because the url rewriting is not working. Are you accessing Flarum from the url defined in config.php ? Is the .htaccess enabled for the website ?
