I have viewed a few of your guitar galleries on my own iOS device (Mobile Safari, iOS 11.3.1) and all the ones I've viewed display and function OK (without any error messages).
If the problem happened "out of nowhere" (were the galleries were previously working OK but suddenly stopped working without you having changed anything yourself), then it sounds like something may have happened on your web server to cause the problem.
As everything seems to be OK at the moment (at least for myself), then it sounds like the problem may have been a temporary glitch on your server which has now been resolved.
If you are still seeing the "Config file not found." message, then try clearing your browser's cache before reloading your web pages to see if this helps.

Also, please note that the Juicebox module for Drupal is an unofficial plugin which we did not write ourselves and it is the module's own custom code which generates the config files and the structure of the galleries. As such, if you continue to experience difficulties, it might be worth posting in the Drupal forum where the author of the module should hopefully be able to help you out further.

I hope this helps (or at least points you in the right direction).
Please let me know how you get on. (Hopefully the problem was just a temporary server glitch which has now been resolved.)