Synchronous loading causing warning

Synchronous loading causing warning

I am developing the jQuery plugin below, and this  issue at GitHub was raised by a user .

The plugin is responsible for "throwing" a warning in the console:

Synchronous XMLHttpRequest on the main thread is deprecated because of 
its detrimental effects to the end user's experience.
I attempted to research the possible cause and have ruled out that it is due to a missing

  1. async: true

in the central $.ajax() call.

Next, I've ruled out, that it is due to attaching script tags, as described in here...

I'm now clueless, and wondering what else might be a classic reason for this warning message?

Thanks in advance!

jQuery Ajaxify Plugin: