Hi,
as shown on Drupal issue #2540598 is necessary to change the class and method used in fullcalendar_legend.theme.inc from String::checkplain() to Html:escape() to fix:
Fatal error: Cannot use Drupal\Component\Utility\String as String because 'String' is a special class name in /var/www/publicpower/web/modules/contrib/fullcalendar/fullcalendar_legend/fullcalendar_legend.theme.inc on line 8
Hi, as shown on Drupal issue #2540598 is necessary to change the class and method used in fullcalendar_legend.theme.inc from String::checkplain() to Html:escape() to fix:
Patch provided on issue page