HTML5 Drag&Drop Not Working When Served

Hi! This is a bit of a long one, so bear with me...

I have a number of projects that were originally authored in Storyline 1 with HTML5 output, and verified to function correctly in all browsers, both as SCORM 1.2 content hosted in an LMS, and as unscored web content hosted on a standard web server. Recently, it has come to my attention that these now show white screens inside the player window (presumably due to browser upgrades), so I went back to re-author in the latest version of Storyline 1, but that produced the same results.

I then tried to upgrade the projects to Storyline 2 (latest version). There were some behavior issues with the converted output, but I was able to work through most of those and produce a published file (with HTML5/ unscored) that does work when launched locally (and verified it was the HTML5 version running). When loaded to a web server (not LMS), however, after the initial build-in animation, an additional "wait" spinner appears. When the spinner finishes, the dragging functionality (freeform) works, but drag-over and drop-on functionality does not work.

I've tried every variation of layer/object options I can think of and copy/pasting into a new Storyline 2 document, with the same results. I've tried to port the project to Storyline 3 as well, but functionality is even worse there.

I'd be happy with any path that gets me to useable HTML5 output - basically where I was when the project was first completed and verified. I can privately share the project for examination, so if you'd like me to send it somewhere, just let me know.

Thanks in advance for your help!!