Help needed diagnosing broken slide(s) in Storyline Web Build

I'd greatly appreciate any help the Storyline Help folks can offer with this issue: I appear to have a slide or two that are breaking the Web Build of my entire storyline project. I have a slide (Slide #3) that has a video playing in it. When I click the NEXT button to advance to Slide #4, the storyline project breaks in a browser-specific way, described below.

In Chrome:
On Slide 3 when I click the NEXT button, the slide does not advance. Instead, the video on slide 3 continues playing and a semi-transparent gray film appears over the video with three moving white loading dots. The video continues playing in the background, but all of my navigation buttons (BACK, NEXT, and RESTART) are unclickable, as are the seek bar buttons (volume, pause, and replay). Because of this, I am essentially locked into the slide, with no way to advance forward/back. See screenshot1.jpg

In Firefox:
On Slide 3 when I click the NEXT button, the Slide *appears* to advance (I say *appears* because if you watch the seek bar progress, it snaps back to the beginning as it typically would at the start of a new slide) *but* the content I see is not an actual slide I've made. I see a textbox from Slide 3 that reads "PATIENT EXPERIENCE SPECTRUM", though the text box is placed oddly in the upper left corner. A blue rectangular box also pops up a few seconds later, and it kind of looks like the "I AGREE" button from Slide #4. My navigation buttons (BACK, NEXT, and RESTART) aren't there at all, so I can't move forward/back. The seek bar buttons (volume, pause, and replay) ARE there/visible and they work correctly when I click on them. See screenshot2.jpg

Important notes: Although we are hosting this Web Build on a Wordpress site, we are still experiencing the problem when we access the story_html5.html file directly from our FTP. It thus seems unlikely that Wordpress is the culprit. Another detail: I've had other coworkers test out both the hosted version of the Web Build AND directly from our FTP and the result is consistent: they are experiencing the problem on their first view.

Here are the circumstances where the slide advances successfully without this breaking issue:
- in private mode for Firefox
- in incognito mode for Chrome
- Microsoft Edge (both Standard mode and Private mode)
- when I open the web page from my local story_html5.html file (directly from the Web Build folder on my local computer)

Attached are the two screenshots I mentioned above, as well as the packaged Web Build and the .story file. In case it matters, I am working on a native Windows machine (not using a VM or any kind) to make this storyline project. The problem is appearing on Macs/Windows alike. And finally, here is our iframe code, just in case: <iframe src="https://Nuvoedge.com/Articulate_Projects/articulate_test/story_html5.html" width="980" height="658"></iframe>

Many thanks for your help on this one!