A ruleset is created containing rules to be executed at a specific time. This ruleset is scheduled by a reaction rule.
The trigger is when a new user is saved.
When a new user is saved the scheduled task is created and can be found in the rules_scheduler table. Now, executing Cron manually brings the following error message:
Fatal error: Call to a member function restoreBlocks() on a non-object in /var/www/drupal7/sites/all/modules/rules/rules_scheduler/rules_scheduler.module on line 62.
When Cron is executed before the task is scheduled it remains in the table (as it should be).
After its scheduled time it is deleted from the table.
Nevertheless, in both cases this problem occurs.
The task itself was not executed at all.
Drupal v7.14, Rules/Scheduler 7.x-2.1.