But, well, it worked until yesterday and I was able to expand an object with gradient, but now it doesn't work. (I have a little bit strange things to do, where the shape with radial gradient without expanded object looks very bad, so that's why I'm asking what's the problem in AI)
You shouldn't have the Object option available unless the selected object contains something more complex, such as a symbol, envelope, or blend. As I said, if there's nothing but a gradient fill, there is literally nothing else to expand. What are you trying to accomplish that would somehow need to be any more expanded?