The WordFence plugin can sometimes cause an issue with Visual Composer post-editing or downloading of assets. It majorly occurs due to a false trigger from WordFence to block such operations. To handle it add following values under: WordFence - All Options - Whitelisted URLs.
URL
Param
/wp-admin/admin-ajax.php
request.body[vcv-content]
/wp-admin/admin-ajax.php
request.body[vcv-elements-css-data]
/wp-admin/admin-ajax.php
request.body[vcv-zip]
/wp-admin/admin-ajax.php
request.queryString[vcv-action]
/wp-admin/admin-ajax.php
request.queryString[vcv-admin-ajax]
/wp-admin/admin-ajax.php
request.queryString[vcv-zip]
/index.php
request.queryString[vcv-ajax]