I understand, but to help more information is needed.
Different WordPress versions act differently, as do their plugins, so does Web Server running (Apache, Nginx, LightSpeed) and it's compiled modules.
Then you get into PHP versions and options.
Then Ajax.
Even they way different control panels, since they handle the permissions.
A core error could narrow it down to a WP config.
But without knowing details about the script, and current config, it's going to be hard to try & duplicate.