The FullCalendar Options and FullCalendar Colors are not D9 compatible.
Error on using any of the modules:
User deprecated function: Theme functions are deprecated in drupal:8.0.0 and are removed from drupal:10.0.0. Use Twig templates instead of theme_fullcalendar(). See https://www.drupal.org/node/1831138 → in Drupal\Core\Theme\Registry->processExtension() (line 498 of /app/core/lib/Drupal/Core/Theme/Registry.php)
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