Why does my Vaillant boiler shut itself off every day?

I’m subtenant and I have Vaillant boiler (don’t know which model). Every time I use shower after few minutes boiler shuts itself down and the burner lockout indicator is flashing. I’m then without hot water and central heating.

When it shuts down, I start it by pressing the red button. Some times it only provides water to run the central heating.

I’ve called a repairman and he couldn’t solve the problem, he even asked 2 of his colleagues, they couldn’t solve the problem either. Here is the picture of boiler and burner lockout indicator: