Captivate 2019 Published HTML5 displays as only white screen on server.

I've published a quiz course in a non-scalable HTML5 format. The un-zipped files load successfully into any of the browsers I use locally on my machine. It also presents correctly locally on other co-workers' machines. 

However, when uploaded to a directory on our customer-facing server, navigating to the screen shows only a blank, white page - not even the "play" arrow. This is not for actual use yet, but simply customer preview for a version they will host on another server when we do the final publish. No score recording needed.

I've checked file permissions, firewall settings etc. but have not found any solutions or clues. I've double-checked that all files successfully upload. 

I'm probably missing something obvious and would welcome anybody that can help me find what is probably right in front of my nose.