Well, in his last message, Valerio pointed out that the failure has been in the ABS, NOT in the rear brake.
Since it seems that the ABS system is split in two, front/rear, he just had to bypass the rear part to get back the rear brake functionality.
Thus it's not a matter of debris.
He also underlines that the workaround he did is not possible on MY2010 and later because of the combined braking sytem.
He thinks that this is a serious problem that can arise and hopes that it won't replicate to others.
(Actually he used more "colorful" words, but I smoothed them a bit...

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