If I understand well, you bought it from Kemper Cycles in The Netherlands ?

Its a EU shop, in the EU, the seller is fully responsible even if the fault comes from the supplier or manufacturer.

They should ship you a replacement rear light at their costs and do the custom paper work to avoid you have to pay additional import costs (they can demonstrate that its a replacement part for an already imported one).

Now, having worked many years in a big dutch international company with many dutch colleagues, I know dutch people are hard in business, its difficult to make them pay anything, everything in a commercial transaction should be clearly written so that it can later be proven!