Storyline 360 content sometimes not loading properly?

Hi everyone,

I've been developing in Storyline since the SL1 days, but have recently run into something new-to-me with a client. Sometimes text isn't rendering properly. This course was built int SL360 and is HTML5. I serve the course to them through Backblaze B2. The problem is intermittent and doesn't always happen, even on the same computer when loading the course at different times.

Here is what they saw one time when administering the course:

Here is what the slide looks like when rendered properly:

They use IE 11 on Windows 10 and have a strong internet connection. Does anyone have ideas how to reduce the frequency of this type of error? Thank you!