Currently, the calendar uses the database backend:

The new_event method starts in database_driver and it starts _update_recurring. In this _update_recurring there is a calendar_recurrence object that creates Horde_Date_Recurrence on initialization where the problem is. :)

Yes, it using plugins/calendar/drivers/database/database_driver.php.

Any info? :) Is there anything I can help you to resolve this bug ?

Did attached screen help you repeat the error?

I think that the screen from the application will explain the problem better :)