[davidmanze]

So, the repair will cost about 440€ :-( I don't see alternatives, since it's still cheaper than buying a new copy. Furthermore the lab will recalibrate everything, so there's a (small?) chance the pre-existing decentering defect will be fixed.

I'm asking for an advice: there are two little cosmetic problems with the focus and zoom rings (small scratches); I thought to take advantage of the repair to have them replaced, just in case I sooner or later sell it when an alternate better lens will be available. Changing them will cost extra 75€. Is it worth while in your opinion?

Well  515 EUR that's practically the price of Sigma 18-50  you recommended us yourself....Just saying...

 That's a stratospheric ouch .. out of the blue like that !!