Debug server error in legacy code

I manage an existing website that runs code that was written 5-6 years ago by someone else who is no longer associated with the project. Recently we ported the system to a new cloud server; seemingly the installation went smoothly, but we received reports from users about problems with the image file upload function, which upon investigation appears to arise within the file jquery.js. (The author of this software chose to include this file in the repository, so it is currently frozen at version 1.11.1). The same software runs without error on another server instance running the same OS (Ubuntu 18.04), so it is a bit of a mystery what might be going on here to produce the difference in behavior. We are looking for someone that is not averse to wading through poorly-written undocumented code to help solve this problem.