Dear Daniele and maxell182,
Thank you for your reply. I finally found the cause of the problem; it was an IP address conflict. When it works, I can use the path \sunrise.box\disk_part\ or \192.168.1.1\disk_part\, both work correctly.
Here’s what happened: when I tried the remote connection, I was connected to a local Wifi network from a Sunrise Box 3 whose IP address was 192.168.1.1. As both Sunrise Boxes (local and remote via VNP) have the same IP address, this can’t work. I have to try changing the IP address of my Sunrise internet box and use another one.