I just got the backup disk back, also btrfs, which was made via send/receive... It has the same errors during fsck.
The main disk still hasn't found any file (apart from a few, others for which none of my hash sums were stored at all) that doesn't verify. So I guess there's definitely some bug in btrfs, that even propagated via send/receive or was so common that it happens on another fs as well; even though it's unclear whether this was just in older versions. Perhaps Qu can shed some light on this, when he had a look at my extent tree dump. Cheers, Chris.
smime.p7s
Description: S/MIME cryptographic signature
