Hi Arif,
The site ajax URL seems to be set incorrectly. The ajax requests are made to the following URL:
https://yoursite.com/ajax
..while it should be:
https://yoursite.com/wp-admin/admin-ajax.php
If you changed this anywhere programatically, I highly recommend swithing it back to the detault, as it seems like some of the requests are not properly registered with it.
Alternatively, you can try turning on the custom ajax handler, that is bundled with the plugin. That usually resolves these types of issues.