PHP Fatal error: Uncaught Error:  operator not supported for string
This error is because of the deprecated code usage in applications.
In PHP 7.1,  operator not supported for strings for various Joomla, WordPress and Drupal components. So it will throw the above error message.
In order to solve the issue, downgrade the PHP version which is less recommended and can choose as a temporary fix.
Better permanent solution is to upgrade the applications with latest code.