100 credits to find the error

[galaxyAbstractor]

            case 'htaccess':
                $this->write_type = 'htaccess';
                $display_vars['title'] = 'ACP_HTACCESS';
                $user->lang['ACP_HTACCESS_EXPLAIN'] .= '<br/><hr/><b>' . $user->lang['ACP_PHPBB_SEO_VERSION'] . ' : ' . $this->modrtype_lang['link'] . ' - ' . $phpbb_seo->version . ' ( ' . $this->modrtype_lang['forumlink'] . ' )</b><br/><br/>';
                $display_vars['vars'] = array();
                $display_vars['vars']['legend1'] = 'ACP_HTACCESS';
                $display_vars['vars']['save'] = array('lang' => 'SEO_HTACCESS_SAVE', 'validate' => 'bool', 'type' => 'radio:yes_no', 'explain' => true,);
                $display_vars['vars']['more_options'] = array('lang' => 'SEO_MORE_OPTION', 'validate' => 'bool', 'type' => 'radio:yes_no', 'explain' => true,);
                $this->new_config['save'] = false;
                $cfg_array = (isset($_REQUEST['config'])) ? utf8_normalize_nfc(request_var('config', array('' => ''), true)) : $this->new_config;
                $this->new_config['more_options'] = isset($cfg_array['more_options']) ? $cfg_array['more_options'] : false;
                $this->new_config['slash'] = isset($cfg_array['slash']) ? $cfg_array['slash'] : false;
                $this->new_config['wslash'] = isset($cfg_array['wslash']) ? $cfg_array['wslash'] : false;
                $this->new_config['rbase'] = isset($cfg_array['rbase']) ? $cfg_array['rbase'] : false;

                if ($this->new_config['more_options']) {
                    $display_vars['vars']['slash'] = array('lang' => 'SEO_HTACCESS_SLASH', 'validate' => 'bool', 'type' => 'radio:yes_no', 'explain' => true,);
                    $display_vars['vars']['wslash'] = array('lang' => 'SEO_HTACCESS_WSLASH', 'validate' => 'bool', 'type' => 'radio:yes_no', 'explain' => true,);
                    $phpbb_path = trim($phpbb_seo->seo_path['phpbb_script'], '/');
                    if (!empty($phpbb_path ) && !$phpbb_seo->seo_opt['virtual_root']) {
                        $display_vars['vars']['rbase'] = array('lang' => 'SEO_HTACCESS_RBASE', 'validate' => 'bool', 'type' => 'radio:yes_no', 'explain' => true,);
                    }
                }
                // Dirty yet simple templating
                $user->lang['ACP_HTACCESS_EXPLAIN'] .= $this->seo_htaccess();
                
            break;
            default:
                trigger_error('NO_MODE', E_USER_ERROR);
            break;

Something in there is causing a 400 bad request on x10s servers. What?
Edit:
fixed