I do appreciate you taking the time to respond so quickly and to investigate and test my site. Nonetheless, it is somewhat deflating to be told “it works for everyone else”. I accept that if it is not a theme-related issue, there is nothing more you can do.
If anyone else reading this has any suggestions, I’d be glad to hear what else I could try. I have the latest versions of WordPress (reinstalled, just to be sure) and the theme.
In the meantime, I have just decided to reduce the width of the sidebar site-wide (with custom css). Not ideal, but will serve the purpose of giving me a wider page.