Minecraft on Android - can't mine blocks?

I installed MInecraft (demo) on my Android phone for my son. After a while the following problem arises:

You can't mine blocks or beat monsters. They are "selected" (lighter) when tapped, but nothing happens. Reinstalling will fix the problem, but it will come back after a short while. At one point I managed to restore normal function by any movement of my finger, but again not for long, and I'm unable to repeat this.

I suspect that somehow we accidentally turn on adventure mode, but how? Going to the chat console every time and changing the mode is not a solution.

I would buy the full version if I knew for sure that the error did not occur there, but not that way.

Have a look in the settings of the world, which mode you have set there.

Otherwise cheats on and change the mode.