@Stefano I am running Ubuntu MATE 17.04 on my PC. I've also tried other distros, including Linux Mint and MX Linux to rule out an OS-issue.
I've tried booting my phone into fastboot mode and my PC doesn't detect it as a device then. Haven't tried in Recovery mode.
Update: I looked up the instructions for recovery mode, but it requires that the PC can detect the phone in fastboot mode, which doesn't work for me. My OSes have no indication there is a device plugged in, even when using "lsusb" to see all connected devices.
If I'm going to update, I think it will have to be by editing something on the phone itself to get it to recognize UBport's updated images.