Thank you for the links to your galleries.
I see that you are using a custom PHP file (named 'config.php') to dynamically generate your gallery's XML data.
The problem lies somewhere within this file (which is not part of the Juicebox package).
There is something within this file that is not compatible with PHP 7.2 onwards.
Unfortunately, I do not have access to your 'config.php' file to inspect its code but opening it directly in a browser reveals the following warning:
Warning: count(): Parameter must be an array or an object that implements Countable in config.php on line 10
Your use of the PHP count() function (whatever it may be) on line 10 is listed in the 'Backward incompatible changes' on the 'Migrating from PHP 7.1.x to PHP 7.2.x' support page here.
An E_WARNING will now be emitted when attempting to count() non-countable types (this includes the sizeof() alias function).
In order for your 'config.php' file to be compatible with PHP 7.2 onwards, you'll need to find a new PHP method to count whatever it is that you are counting.
I hope this points you in the right direction.
If you continue to experience difficulties, then please zip and upload your 'config.php' file somewhere (and provide a download link) so that I can inspect the code and hopefully help further.