Found the solution:

adb reboot bootloader

Then boot it into recovery and then:

ubuntu-device-flash touch --device=mako --server= --channel=15.04/stable

This ends with:

2018/02/26 21:42:20 error pushing: adb: error: connect failed: no devices/emulators found

But after rebooting it it is up and running 15.04 without any problems 🙂

Sorry for not trying harder before posting above!