Hello, @leanazar,
Please, provide FTP access to your site and server error logs.
I see that backend editor is also broken but once I disable the Massive addons plugin it works. We found that other users also faced this issue https://codecanyon.net/item/massive-addons-for-visual-composer/14429839/comments?page=36&filter=all#comment_14143143
Try to follow plugin author suggestions and
First of all, please increase the memory limit do at least 128M (256M is recommended).
Then, please make sure that your server is not using mod_security or FastCGI handler for PHP. If it is, you need to contact your hosting provider and ask to increase the maximum body length value.
Regards