Can't connect to URL "". Please check site's firewall settings


I tried to connect my website to and I get this error:
"Can't connect to URL "". Please check site's firewall settings."

From Convertful support I received this reply:

"This error may happen because our server can't fetch data from your site.
It seems like the plugin is blocked by some 3-rd party plugin.
If you're using any plugins for script minification, optimization, or caching plugins, open such plugin's settings and create an exception for "Convertful.js" in it. After this, clear this plugin's cache and try again."

In the plugin under the "Cache" tab I added this
/index.php?rest_route=/convertful/v2/get_info/" to Excludes -> Do Not Cache URIs

Can you help me with a check or guidance where I could add as an exception "Convertful.js"