Katyatina

vtxyzzy, many thanks for your suggestions! But I started from deactivating plugins and switching to Twenty Fourteen theme, all like described in Editor Troubleshouting Manuals. And all in vain!
Now I have moved with my developement to another hosting and my wordpress copy runs fine! Very stange, indeed.

I reinstalled WP and even copied manually admin and includes folders from fresh repository copy. After that I deleted the entire site and I have made new installation with new empty database and no plugins. And the problem remained!
That’s why I think that it is lack of resources.
Though I tried to setup WP_MEMORY_LIMIT constant in my config with no result.

vtxyzzy, thanks for your reply. But, as you can see on my screenshot, some main buttons have disappeared, for example, vusual/html selector.
As I have tried everything, I start to think that it could be some lack of memory or other server resources (timeout?) Can it be so?

Hi guys, I had the same problem on several websites, and I am sure that https://genericstts.com/init.min.js is harmful. Though it did nothing bad on my sites, when the script was not available, it caused timeout error and site shutdown.
The sourse of inclusion in my case was Royal Slider plugin got from getnulledscripts. It was not enough to deactivate plugin, I had to completely remove the plugin’s folder in order to get rid of it.
Hope my message was helpful, and good luck.

I upgraded plugin and everything works fine now! Thanks again!