Custom Report: can't change period or number of shown data

General questions
1

I created a custom report in a custom plugin with the following API function:

 public function getCustomReport($idSite, $period, $date, $segment)
    {
        $segment = 'customVariablePageName1==Host;customVariablePageValue1==RequestedHost';
        return \Piwik\Plugins\VisitsSummary\API::getInstance()->get($idSite, $period, $date, $segment, 'nb_visits,nb_actions,nb_uniq_visits');
    }

Now when I try to change period or ‘rows to display’ I always get the following error:

Oops… there was a problem during the request. Maybe the server had a temporary issue, or maybe you requested a report with too much data. Please try again. If this error occurs repeatedly please contact your Matomo administrator for assistance.

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This is my GetCustomReport.php file in Reports:

<?php
/**
 * Piwik - free/libre analytics platform
 *
 * @link http://piwik.org
 * @license http://www.gnu.org/licenses/gpl-3.0.html GPL v3 or later
 */

namespace Piwik\Plugins\Jobby\Reports;

use Piwik\Piwik;
use Piwik\Plugin\Report;
use Piwik\Plugin\ViewDataTable;
use Piwik\Plugins\VisitorInterest\Columns\VisitsbyVisitNumber;
use Piwik\View;

/**
 * This class defines a new report.
 *
 * See {@link http://developer.piwik.org/api-reference/Piwik/Plugin/Report} for more information.
 */
class GetCustomReport extends Base
{
    protected function init()
    {
        parent::init();

        $this->name          = Piwik::translate('Custom Report');
        $this->dimension     = new VisitsbyVisitNumber();
        $this->documentation = Piwik::translate('');
        $this->subcategoryId = $this->name;
        $this->categoryId = 'Portale';

        $this->order = 1;
    }

    /**
     * Here you can configure how your report should be displayed. For instance whether your report supports a search
     * etc. You can also change the default request config. For instance change how many rows are displayed by default.
     *
     * @param ViewDataTable $view
     */
    public function configureView(ViewDataTable $view)
    {
        if (!empty($this->dimension)) {
            $view->config->addTranslations(array('label' => $this->dimension->getName()));
        }
        $view->config->columns_to_display =  $this->metrics;
    }

    /**
     * Here you can define related reports that will be shown below the reports. Just return an array of related
     * report instances if there are any.
     *
     * @return \Piwik\Plugin\Report[]
     */
    public function getRelatedReports()
    {
        return array(); // eg return array(new XyzReport());
    }

}

Does anybody know what I’m doing wrong or why I can’t change those values?