If a Control Rig is added to the BP after 25.2, the Control Rig Class does not allow selection of a previously created Control Rig component. After selecting the Control Rig component in the Content Browser, the Control Rig component can be set in the BP (Control Rig → Control Rig Class → Use the asset selected in the Content Browser). In this case, the session cannot be started due to an error.
@T03 Thank you for your report! We would like to look into this further, would you be able to submit a bug report using the form available here? Fortnite Creative
Looks related to the issue I’m having.