Suddenly z-wave devices become unknown

I have some very strange behaviour from my zwave network.

Suddenly, like this morning, several z-wave devices show up as unknown. This time it was approx. 10 of them.
Yesterday evening everything was fine.
But yesterday morning nearly all devices were unknown! That gave me a bit of a shock, as you can imagine.
After a while, it comes back to normal.
When I restart, and wait for 10-15 minutes, it looks normal again.

Is this normal? Is there something I can do to make it stable?
I’m on Debian stretch, Inten NUC, hassio 80.3, Aeotec z-stick, approx. 30 z-wave devices and some philips hue.
Hoping for help from some of you amazing guys!