“There has been a critical error on your website. Learn more about debugging in WordPress.” and web error log reports “Call to undefined function the_field()” [Solved]

After an upgrade of WordPress, the site reported “There has been a critical error on your website. Learn more about debugging in WordPress.”. When investigating the web server log files, errors reporting “Call to undefined function the_field()”.

Solution: During the upgrade process, the plugin “Advanced Custom Fields” had been disabled. By activating the plugin, the problem was solved.