@West African Lion are you saying that your Next.js server deployed on Vercel failed to connect to your upstream, Express server via fetch? but it works localhost -> express API?

Resolved the issue already, had not clarified a sub directory Vercel was supposed to deploy from

But yes

404 means the problem is located in Vercel app. if it resides btw upstream, it returns 502 or 504, bad gateway or timeout. or 5XX in fetch command