I think the problem with admin route or rewrite url in your site (use .htaccess file). Please try to rename the file .htaccess in the webroot in your site, then check the pages builder with widgets again. If it resolved the problem, please try to update the .htaccess to right.
If not, please provide me the site information again. I will help you update the module ves pages builder
Answer for Ves Tattoo Theme, one pager editor widget giving errors