App is working with vite serve but not with vite build | Vite Land | Page 1

I'm migrating a large vue2 app from vue-cli/webpack to vite.

So far, vite serve is working perfectly, but when I build and then load the root page, all I get is this error:

index-8688fdea.js:169 Uncaught TypeError: o.extend is not a function
at oqe (index-8688fdea.js:169:2401)
at index-8688fdea.js:169:2868
at MV (index-8688fdea.js:1623:6042)
at index-8688fdea.js:1623:8992
at index-8688fdea.js:1623:9055

I don't even know where to begin trying to debug why the build is broken. Any ideas?

vite to serve the app works fine. vite build and then vite preview gives this horrible error when I load the page.

#App is working with vite serve but not with vite build