Hello Vadim,
Finally I was able to log into your server.
I am able to replicate the exact issue that you are facing in your server but I am not able to replicate it in my dev servers.
I am not sure if it is because of the different plugins that you are using or because of the different language that you are using in the dashboard.
Please check your PHP error log to see if there are any error log entries. If yes then let me know and I can try to fix the issue.
If you have access to another WordPress install then please try out the add-on there as well to see if you are able to replicate the issue there as well.