Fluent CRM- mail sequence is returning with critical error

FluentCRM is expecting JSON data but HTML returned

Please check the browser console -> Network tab and see what's went wrong. Most probably a plugin conflict.

I tried both Chrome and safari, all the same issue. Have not done any plugin updates, it just happened yesterday. Any other possibilities?

I had the same issue and it was server cdn. I am using hostinger as a webserver and i reached out to their support and ask if they can disable cdn for testing purposes and since then its working nice.

Basically your firewall or cdn is blocking json file. I will start from disabling any security plugin to see if that solves the problem. If not reach out to your hosting company and ask them if they can disable firewall for testing purposes