I replied to your other thread, but the post has gone AWOL, despite showing me as the last poster. 

Anyway, I shall try again.

Short version: probably not repairable, never heard of this failure before.

Longer version: When ours failed, it cracked at the 90° corner where the plastic stub for the top hose joins the main plastic endcap. Couldn't get any sort of glue or sealant to hold.  Later found that the HG was causing overpressurising, so it may have been fixable if I'd sorted that first, but hey ho.

I took the old rad apart a bit out of curiosity, and took a couple of photos.  The main alloy bit has fingers that crimp over/round the plastic endcaps, as you'll have seen.
In this pic, I think the white-coloured rectangle is probably some sort of compliant rubbery stuff that the plastic endcaps seal against, under compression from the crimped-over fingers:


The endcap here shows some lateral distortion (lower edge in pic), possibly caused by overheating/overpressuring before the HG fault was identified.  Yours may have suffered similar? 

Shouldn't be too expensive to buy an OK aftermarket rad in Germany, I'd think/hope.