The Lightroom plugin v1.2 came bundled with Juicebox v1.2.0 so, if you upgraded the plugin to Juicebox-Pro around that time period, then the version of Juicebox that your gallery is using is almost certainly the cause of your problem.
You can determine the version of Juicebox that a gallery uses by opening the gallery's 'jbcore/juicebox.js' file in a plain text editor and checking the version number in the comments at the top.
You can do likewise with the plugin's 'juicebox.js' file ('juicebox.lrwebengine/jbcore/juicebox.js') to determine what version the plugin is currently using.
As you will likely need to upgrade the plugin's core Juicebox files, you might like to take this opportunity to upgrade the plugin, too.
(1) Download the latest version of the plugin (v1.4.2.0) from here.
(2) Download the latest version of Juicebox-Pro (v1.4.2) using the link from your purchase email (see here for details).
(3) Upgrade the Lightroom plugin following the 'Upgrading to Juicebox-Pro' instructions on the plugin's support page.
If you want to upgrade an existing gallery to the latest version of Juicebox-Pro, then either recreate the gallery with the upgraded version of the Lightroom plugin or replace the 'jbcore' folder in the existing gallery folder with the 'jbcore' folder from the Juicebox-Pro v1.4.2 download zip package ('juicebox_pro_1.4.2/web/jbcore/').