shutdown – Ubuntu Core 18 Raspberry PI 3 B +. No restart

I have successfully installed the latest image of Ubuntu Core 18 on my Raspberry PI 3 B +. It is also possible to ssh the Raspi from another machine, browsing the operating system, even by installing snapshots. So, I can say that the system works.

The problem is that it is impossible to restart. If I ssh the restart command, the Raspi stops properly, but once off (the two red static lights), it can no longer restart alone.

Even if I try to connect it by hand, the rainbow screen appears, then it freezes. After that, to restart, you must copy the image and start over.

I think I'm missing something, so any help will be welcome,