Thanks Peter! Unfortunately it didn’t help. In the suggested portfolio.php file I now have:

if(!empty($title))
                    {
                        $markup = avia_markup_helper(array('context' => 'entry_title','echo'=>false));
                        $output .= '<header class="entry-content-header">';
$target = !empty($custom_link) ? "target='_blank'" : '';
$output .= "<h2 class='portfolio-grid-title entry-title' $markup><a href='{$title_link}' $target>".$title."</a></h2>";
                        $output .= '</header>';
                    }

Is that how it should be? I have only set a title and an external custom link for my portfolio items. When I click on the title of the portfolio item on the frontend, the target external page still opens up in the same window. Any suggestions?

Additionally, I have some items that do not have a link and for these I would like to display only the title on frontend without linking it anywhere externally or internally. Is there a way to achieve this? Right now an empty portfolio item page opens up when I leave the “Link portfolio item to external URL” empty.