I have created a staging site so as to not harm the live site while troubleshooting (credentials in hidden content area).
If it helps any, this issue occurred when we updated the Basel theme from version 5.6.1 to version 5.9.3.
No plugins were added or changed during the theme update, so the update itself is directly involved.
As an aside, the update also caused the shop page to turn blank — an issue that seemed to arise from a change in the “product categories” widget after the update. We remedied that by manually adding each category to the widget, which resulted in the page populating properly once again.
So it appears to be the latest theme update at the center of whatever is going on, and not a plugin issue.
Please take a look at the staging site and let me know what your thoughts are.
Thank you