I had the same issue! Dead left side. Not the cable, but the headphones themselves. I've contacted support, will update when I hear back. Really disappointed in this.
Really sorry to hear
I'm from Denmark too and will no longer accept customs taxes, non QC and need for hassly support
Next time going local or inside EU
HiFiMAN support e-mailed within 24-48 hours. They game me an RMA# and interactions on how to send it back and offered a full replacement. Downside - I have to pay for shipping; Upside- they offered me a pair of RE300 for the trouble. I thought the effort on their part to make it right was admirable. I'm excited to get the new pair of HE4XX's, hopefully without issue this time.