I don't know why the problem happened in the first place (it sounds like updates to Chrome and Edge might have prevented redirects within iframes from working correctly) but I'm glad that my suggestion worked.
Thank you for letting me know.
Incidentally, just in case you are not aware, you can embed Juicebox galleries directly into WordPress pages and posts (without the need to use iframes) by using the baseUrl method of embedding documented here.
Essentially, once you have created a Juicebox gallery on your computer (perhaps with JuiceboxBuilder), you would upload the complete gallery folder to your web server and paste the baseUrl embedding code into the body of your WordPress page or post (ensuring that the method of entry is 'Text' rather than 'Visual'). It does not matter where on your web server you upload your gallery folder to as long as the two paths in the embedding code (the path to the 'juicebox.js' file and the baseUrl itself, pointing towards the gallery folder) are correct.
Also, there is a dedicated Juicebox plugin for WordPress called WP-Juicebox which would allow you to create and manage galleries from within the WordPress dashboard. The plugin handles the embedding of the galleries automatically.
You can download WP-Juicebox from its support page here.