I exported a macOS application that uses save() to export png files to the application’s base directory (parallel to the app file).
It works fine while running in processing.
It works fine as an application running on the machine where it was created.
It works fine if I transfer it to another mac over the network.
But, if I upload it to a web server and download it on another mac, it will run but the save() function seems to have no effect.
I found the problem and, thankfully, I didn’t need to pay $100 to apple and go through the app signing process!
In case someone else has a similar problem, I was exporting the application, which creates a folder in ~/Documents/processing/<project_name>. I was zipping that folder, containing the executable and source files and uploading it to a web server.
When downloaded from the internet, the exported application would not save() a PNG file inside that folder (worked fine if transferred over the network).
I moved the application file outside of the folder and ran it there and the save() function works fine.