Thanks for updating the thread.
I tried taking a look at the setup and so far I am not able to find out the issue. Further I also tried to install debug plugin along with taking a look at the FTP setup and don't see anything unusual.
In that regards, I suggest you to try out following:
- Please update WordPress to 5.1.1. I see that you are using old version of WordPress. Please update WordPress.
- Please try testing for plugin conflict by deactivating all plugins and then reactivating one at a time to narrow down. I tried to deactivate Wordfence but that didn't help. Further I need a permission from your end as it's a live website and I am not allowed to test for plugin conflict in depth without taking permission.
Let us know how it goes.