Where is this file upload error happening?

I have a webapp that works perfectly well for me that has the ability to upload a file and visualise some outputs based upon its content. A colleague has come across an issue trying to upload a file that works for me with the following error. I'm trying to figure out where the error is happening so that I can try and fix it. My guess is that something, perhaps a firewall, is blocking the uploading of the file so that it never makes it to the subsequent processes, but is there anything I can use to help figure it out and solve it? I've never had to ask a question before, so if I need to provide more info then please let me know.