Report this upstream when calendar is not a prototype anymore and the bug still exists.
To reproduce, create a new task, set no invitees and make it recurrent. Maybe the start and end date need to cover today (untested).
[04-Apr-2017 16:18:07 UTC] arcanist(head=stable, ref.master=3b6b523c2b23, ref.stable=60aaee0ed3f5), phabricator(head=stable, ref.master=1953ab98be2b, ref.stable=2460755603cb), phutil(head=stable, ref.master=91ab940c3979, ref.stable=f568eb7b9542) [04-Apr-2017 16:18:07 UTC] #0 qsprintf_check_type(array, string, string) called at [<phutil>/src/xsprintf/qsprintf.php:134] [04-Apr-2017 16:18:07 UTC] #1 xsprintf_query(AphrontMySQLiDatabaseConnection, string, integer, array, integer) called at [<phutil>/src/xsprintf/xsprintf.php:70] [04-Apr-2017 16:18:07 UTC] #2 xsprintf(string, AphrontMySQLiDatabaseConnection, array) called at [<phutil>/src/xsprintf/qsprintf.php:64] [04-Apr-2017 16:18:07 UTC] #3 qsprintf(AphrontMySQLiDatabaseConnection, string, array) called at [<phabricator>/src/applications/people/query/PhabricatorPeopleQuery.php:292] [04-Apr-2017 16:18:07 UTC] #4 PhabricatorPeopleQuery::buildWhereClauseParts(AphrontMySQLiDatabaseConnection) called at [<phabricator>/src/infrastructure/query/policy/PhabricatorCursorPagedPolicyAwareQuery.php:281] [04-Apr-2017 16:18:07 UTC] #5 PhabricatorCursorPagedPolicyAwareQuery::buildWhereClause(AphrontMySQLiDatabaseConnection) called at [<phabricator>/src/infrastructure/query/policy/PhabricatorCursorPagedPolicyAwareQuery.php:104] [04-Apr-2017 16:18:07 UTC] #6 PhabricatorCursorPagedPolicyAwareQuery::loadStandardPageRowsWithConnection(AphrontMySQLiDatabaseConnection, string) called at [<phabricator>/src/infrastructure/query/policy/PhabricatorCursorPagedPolicyAwareQuery.php:90] [04-Apr-2017 16:18:07 UTC] #7 PhabricatorCursorPagedPolicyAwareQuery::loadStandardPageRows(PhabricatorUser) called at [<phabricator>/src/applications/people/query/PhabricatorPeopleQuery.php:167] [04-Apr-2017 16:18:07 UTC] #8 PhabricatorPeopleQuery::loadPage() called at [<phabricator>/src/infrastructure/query/policy/PhabricatorPolicyAwareQuery.php:236] [04-Apr-2017 16:18:07 UTC] #9 PhabricatorPolicyAwareQuery::execute() called at [<phabricator>/src/applications/calendar/notifications/PhabricatorCalendarNotificationEngine.php:126] [04-Apr-2017 16:18:07 UTC] #10 PhabricatorCalendarNotificationEngine::sendNotifications() called at [<phabricator>/src/applications/calendar/notifications/PhabricatorCalendarNotificationEngine.php:58] [04-Apr-2017 16:18:07 UTC] #11 PhabricatorCalendarNotificationEngine::publishNotifications() called at [<phabricator>/src/infrastructure/daemon/workers/PhabricatorTriggerDaemon.php:478] [04-Apr-2017 16:18:07 UTC] #12 PhabricatorTriggerDaemon::runCalendarNotifier(integer) called at [<phabricator>/src/infrastructure/daemon/workers/PhabricatorTriggerDaemon.php:110] [04-Apr-2017 16:18:07 UTC] #13 PhabricatorTriggerDaemon::run() called at [<phutil>/src/daemon/PhutilDaemon.php:219] [04-Apr-2017 16:18:07 UTC] #14 PhutilDaemon::execute() called at [<phutil>/scripts/daemon/exec/exec_daemon.php:131]