We found that this problem only occurred if the course being uploaded to ftp server and viewing from IE11 on Windows 7; viewing it from IE11/Edge on Windows 10 or any other browser is fine. So we tried upload it to SCORM Cloud and there is no issues with requests being aborted (all responses are 200).

My colleague pointed out that SCORM Cloud is using Apache 2.2.x, and the ftp server we are running has Apache 2.4.x.

It looks like an Apache issue, there is a fix for it if you are going to host the course on a FTP Server: https://stijndewitt.com/2014/01/10/apache-hangs-ie11 (Windows only, Linux also seems to have the same problem but I still couldn't find any solution).

Fix:
Add this configuration snippet to Apache24/conf/httpd.conf (bottom of file seems fine):

# Apparently this fixes an issue with Apache 2.4.6 on Windows hanging
# when serving requests from Internet Explorer 10/11.
# see https://stijndewitt.wordpress.com/2014/01/10/apache-hangs-ie11/
AcceptFilter http none
AcceptFilter https none