Please do not edit theme core files as you will loose your modifications after theme update. The issue on your website is caused by old outdated PHP version installed on your server. Please update the PHP version (contact your hosting company) to resolve the issue. WordPress recommends using PHP7.2.
You’ve also mentioned in our email conversation that you haven’t received any email from this support forum. Well, I think I’ve found the issue: please fix your email address‘ domain (you have typed .der but should be .de?).