Thanks for the shout-out Russell!
It sounds like you have manually migrated the content to S3 rather than relying on this plugin to do the work for you. I'd guess that your manual migration has put the files into a structure/path that the plugin isn't expecting. especially if you have copied your complete moodledata folder into S3 and not just the uploaded user files. (The tool_objectfs plugin does not replace the need for a normal moodledata directory, it just allows the majority of your files to be stored in S3.)
Usually you would have a Moodle site set up with a normal moodledata directory and then you would install our tool_objectfs plugin which would migrate files from moodledata to your s3 storage, relying on the plugin to perform the migration for you.