A huge shout out to @klassenjm in the WordPress.org Forums for providing a solution for some authentication failures that some users were noticing.
In our WordPress.org Support Forum, someone noticed that there is a conflict with mod_security and how WPSiteSync for Content authenticates on the Target server on WP Engine hosting. And the fix was to disable the rule that is getting triggered by adding the following into your .htaccess file on the Target site:
We’re working on some code changes that will allow users to keep their mod_security settings the same without triggering this rule. But for now, if you’d like to use WPSiteSync for Content on WPEngine, this is the work around.
We truly love the WordPress.org Community. A special thanks to @klassenjm for a quick and elegant solution. The development contribution is incredible. Please continue to provide us with feedback. WPSiteSync 1.0, here we come!