Continuous water flow from concealed toilet cistern - any fix?

https://preview.redd.it/yiacpp8ev55h1.png?width=1111&format=png&auto=webp&s=b1266417038d44254466a8021ab376cc168c27a5

Hi,

I have a problem with a concealed toilet tank (the tank is built into the wall).

The water started flowing continuously into the toilet without me pressing the flush button. After I press the button and the tank is full, the water flows continuously into the toilet in the same manner.

This has happened before, but in that case the problem seemed to be related to the flush button itself. After I pressed it a few times in a certain way, the problem resolved itself.
This time, however, I tried pressing both the button and the wall area where it is mounted, but nothing changes.

Has anyone else had a similar experience? What turned out to be the cause and how did you fix the problem?

Is there any chance that this can be fixed without dismantling anything or will I have to access the mechanism behind the actuator plate?

Thanks!