After the WordPress decided to use so called Gutenberg’s block editor, I didn’t like it and installed a plugin “Classic Editor”. After some update I lost possibility to type anything in the Classic Editor. The problem was the following: I use both protocols on my WP site: HTTP and HTTPS. Default is HTTP. But if you set in the WordPress settings https as default and trying to write a new post usgin HTTP, it will not be possible to type anything in the main post field. So I just replaced https:// to http:// when started to write a post and now I am able to type in the main post field.