Steps to reproduce -
1) Navigate to Case Dashboard.
2) Click on the expand icon of one of the listed case to show recent/upcoming activities.
3) Network Error is encountered "Unable to reach the server. Please refresh this page in your browser and try again."
This is due to a non-static function called statically at CRM/Case/Page/CaseDetails.php. It has been deprecated and will throw a "Notice: Undefined variable: this" in PHP 7.