Hey @mstreck. I know 404 errors can be tricky to solve. I would check the deployment output (can be found on the Source tab in deployment details). That will tell you the file structure for the deployment so you can see the path for each file on the site.
I wrote a 404 debugging guide with more tips. It looks like you might have already followed the Express.js deployment guide, but that’s also worth reading through if you haven’t already seen it. Please give these a try and let me know how it goes!