It seems that Stratagem breaks the Calendar Sitemap generation.
OutOfRangeException::0
#0 /var/www/html/105129/init.php(893) : eval()'d code(47): IPS\Patterns\_ActiveRecord::load(133, 'project_calenda...')
#1 /var/www/html/105129/applications/core/extensions/core/Sitemap/Content.php(186): IPS\calendar\stratagem_hook_calendarEvent::getItemsWithPermission(Array, NULL, NULL, 'read', -1, 0, Object(IPS\Member), NULL, NULL, NULL, true)
#2 /var/www/html/105129/system/Sitemap/Sitemap.php(85): IPS\core\extensions\core\Sitemap\_Content->getFilenames()
#3 /var/www/html/105129/applications/core/tasks/sitemapgenerator.php(39): IPS\_Sitemap->buildNextSitemap()
#4 /var/www/html/105129/system/Task/Task.php(367): IPS\core\tasks\_sitemapgenerator->IPS\core\tasks\{closure}()