Activation for item 1 could not be created: IMEI number is already assigned in another application (TQ00261)

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Basically, this only happens when an invoice has been transformed to a credit note and afterwards again to an invoice.
This error can be ignored, as this activation is nothing that is relevant for the billing process.
The activation and the payments take place within the according provider portal.
The invoices can still be found via "All activation" or "Search serial number".