"Loading..." screen appears - how to determine the cause?

I have uploaded a project to the Canvas LMS (not SCORM) but it has developed the “Loading...” issue, where playback stops after a couple of slides and does not resume.

This issue has happened over the years but we have never been able to pinpoint the cause.  What exactly is “loading”?  And, why does it never seem to complete the loading process?

All we have are the HTML files of the project - the Classic version of Captivate no longer runs on our computers.

I’d hate to throw away years of work so I’d appreciate any insight into what we can look at in the HTML files to try and fix this.