I just posted on the issue you filed, but I'll copy that here in case others are curious:
It looks like JSTOR has just recently started preventing framing. Since Guide on the Side relies on this for its normal functioning, you'll have to switch to popup mode. You can find that in the tutorial settings.
It's not ideal, but that's why popup mode exists.
https://github.com/ualibraries/Guide-on-the-Side/issues/184#issuecomment-496566448Mike