When I type
xscreensaver in the dragonboard terminal I get
command not found
However, I managed to solve the problem.
I went to Preferences -> LXQt Settings -> LXQt Configuration Center -> Power Management -> Idle .
There I saw “Enable Idleness Watcher”, which was turned on, together with “When Idle then -> Nothing”.
When I turned the Idleness Watcher off, the problem was solved.
I guess there is some bug there, that it didn’t actually do “Nothing” but something else, like “Shutdown”.
If you can, let the relevant developer know about it.