So far, I have been unable to replicate the problem you have reported.
I successfully created a Juicebox gallery using WP-Juicebox v1.5.0, sourced by NextGEN v2.1.23.
I then upgraded NextGEN to v2.1.43 and the gallery still displayed fine.
I then downgraded WP-Juicebox to v1.4.4.2 and created a new gallery, sourced by NextGEN v2.1.43.
Again, the gallery displayed fine.
I then upgraded WP-Juicebox to v1.5.0 and, once again, the gallery displayed fine.
At the moment, I am unsure what might be causing your problem.
The problem could possibly be due to a JavaScript conflict between the v1.5.0 'juicebox.js' file and another JavaScript file on your page (a JavaScript file that is not being loaded in my own test pages).
To help track down the cause of the problem, please try the following:
(1) Try temporarily reverting to a default WordPress theme (such as Twenty Sixteen) to see if this helps.
(2) Try temporarily deactivating all plugins other than WP-Juicebox and NextGEN to see if this helps. If it does, then reactivate the plugins, one by one (checking your Juicebox gallery after each one), until you find the source of the conflict.
(3) Try using WP-Juicebox v1.4.4.2 with the Juicebox-Pro v1.5.0 'jbcore' folder and also try using WP-Juicebox v1.5.0 with the Juicebox-Pro v1.4.4.2 'jbcore' folder. This should help to determine whether the problem is related to the WP-Juicebox code or the core Juicebox-Pro code.
Each time you check your gallery after trying the suggestions above, please refresh your browser's cache to ensure that your browser is not using old files.
Unfortunately, as I am unable to replicate the problem (it seems to be unique to your own WordPress setup or server), I cannot try these suggestions myself.
Hopefully this will help to point us in the right direction.