Generation Is in Immutability Mode (code 2522)
The following error occurs while attempting to delete some data in a generation: Generation Is in Immutability Mode. The generation is protected with the immutability and cannot be deleted. The immutability period expires on: [date] (code 2522)
The occurrence of this error means that the generation you tried to delete is under the immutability mode that was enabled in the backup plan retention settings. Immutability uses the Amazon S3 Object Lock feature that allows you to store objects using a write once, read many (WORM) model.
This error has no solution: immutable generations cannot be deleted anyhow until the GFS keeping period specified in the backup plan retention settings expires.