This server is seriously misconfigured.
First of all, by default, your server is outputting
Content-Type:application/x-www-form-urlencoded for PHP output.
Content-Type:application/x-www-form-urlencoded; charset=UTF-8
It should not be url-encoded, especially since you are using Cyrillic characters. It should simply be:
Content-Type:text/html; charset=UTF-8
I managed to fix it for the panel MENU (only), by forcing the above content-type from the PHP file. It doesn't seem this can be controlled by .htaccess.
Next up, server is outputting
charset=windows-1251 for other PHP requests:
Content-Type:text/html; charset=windows-1251
Response should be
charset=UTF-8, especially since you are using Cyrillic! I think the above is the
MAIN PROBLEM. We already set UTF-8 from the X3 .htaccess file, but it seems like your server is ignoring this or overriding it.
To further complicate issues, I see this in response headers:
Huh? Your server is NGINX? On your
diagnostics page, it reports "Apache server". This doesn't make sense, and is highly suspect. Do you also have some nginx-specific config somewhere?
Related to this, for your main website document, it returns these values in response headers:
Etag:"c-78060f3189-3122dbe0e1"
Server:nginx/1.10.1
Again, it reports NGINX. Besides, Etag should not be there if the rules from X3 .htaccess are applied properly. Looks to me like either some/all .htaccess rules are ignored and/or you have specific custom server config added somewhere else. Perhaps someone tried to make X3 work with NGINX? That would be possible of course, but some things are not working correctly. Most interesting though, is that response reports "NGINX", while your server reports "Apache/2.4.17 (FreeBSD) PHP/7.0.0".
All in all, this server is simply not configured as standard hosting should be. You will need to contact your host, and feel free to reference the details above. I also had a lot of problems loading panel pages ... I had to refresh several times to get things loaded.
I would be happy to help out if someone is trying to configure this for NGINX.