That looks like a bug in the events component (possibly, specific to Joomla). The error is happening in the constructor of an object that is managing the event registration process (https://github.com/civicrm/civicrm-core/blob/master/CRM/Event/StateMachine/Registration.php), presumably after the payment has occurred. It's failing to get the id of the event, ...


You could try CiviDiscount which offers options for eg auto-discounts for people logged in or using a checksum who are Members

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