Classic web not working - vNext working.

Hi!
We have a customer who initially used Classic Web. Later, we switched them over to New Web only. Recently, they requested that both Classic Web and New Web should be available at the same time.

We followed the official M-Files guide for setting up Classic Web and New Web side-by-side. However, we encountered the following issues in sequence:

  1. 404 Not Found when accessing Classic Web

  2. "You don’t have permission to view this directory or page"

  3. "Server Error in '/' Application" 

At one point, we were able to disable vNext and run only Classic Web, but now Classic Web does not work at all.
Probably broke something during our attempts to fix it.

We would appreciate any tips or guidance on how to resolve this.

Reference:
We have used the guide: Setting up both Classic Web and Web clients breaks Classic Web