I think it's because the runtime:"nodejs", since when I replaced my middleware with the default one from the next docs, the app ran normally. The better-auth docs said that this runtime is supported in next version 15.2.0 above (I'm using 15.3.x). So, why is this happening? What did I miss?

Yes the error is most likely occured because you use the runtime:"nodejs" in Next.js stable version not the Canary one as Next.js official docs states "Note: This feature is not yet recommended for production use. Therefore, Next.js will throw an error unless you are using the next@canary release instead of the stable release." (See: https://nextjs.org/blog/next-15-2#nodejs-middleware-experimental)

So... if you want to use the experimental Node,js runtime in Next.js middleware you have to use the Next.js Canary

Alright that explains it all. I'll look for another approach then. Thanks