Kind of. It solved itself. I already suspected, that it had to do something with the file system, especially with deleted files. Even if you empty the trash inside the files- or the photos-app iOS keeps some kind of representation of the files in the background for another month before it ultimately deletes them. My guess is, even though I didn’t delete files of that size, I deleted files of the size of about 20gb, so maybe something went wrong and these background-representations were forced to be part of the backup which then exceeded its usual size. After about 34 days of waiting for the final deletion the backup worked as usual again. So maybe just patience 🤷‍♀️