Hi - I’m sorry to bump this but running into this issue as well.
I’m running an extension off of the boilerplate react template. I am able to run it successfully on the Rig. To upload it, I ran npm run build to create the dist folder. I then compress the dist folder and am able to upload the compressed zip file successfully.
When I go to install it on my channel, the panel fails to load. I get the same 404 error that the OP had.
I’ve tried using “yarn build” instead of npm and have checked the panel.html files for the helper extension script: <script src="https://extension-files.twitch.tv/helper/v1/twitch-ext.min.js"></script>.
Any help would be appreciated.