3671952 - Not possible to add a new billing due date to a customer project's billing item

When trying to add a new billing due date on a billing item on a customer project, it's not possible due to errors:

• "In the context of Data Services an unknown internal server error occurred"
• "You cannot save the billing data. Please contact support."

SAP S/4HANA Cloud Public Edition

The status of the billing item is 'Completed'.

It's only possible to change billing due dates while the billing item’s status is open.

Managing Billing for Services | SAP Help Portal

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