Could you look for the full error message in storage/logs/flarum.log and/or your webserver logs, or enable debug mode in config.php and try again ?
Without the full error message we won't be able to help ?
Also PS: set HTTPS as your canonical url (in config.php and anywhere you set redirects) and redirect all requests to it (via redirect rules at the webserver level). You even seem to already have the certificate part configured ? Twitter login should work without this change but that's standard web security practice.