What is the max current for a monophase RCCB?
The problem was probably not due to an overload. It looks more like the terminal screw was not tightened properly. There may also have been some damage to the wire that caused the terminal to overheat. Get someone who knows what they are doing to look at the installation. Asking people on the other side of the world to look at pictures on the internet is no substitute for competent workmanship.