In case it helps future users, the issue I had was related to the service user created in MySQL server.
The access rights were too restrictive.
These seems to be fine enough for the appto work:
To help troubleshoot with local debug, remember to set a breakpoint in the catch section of Global.asax.cs. There, you’ll get more details of what the problem is.
It will be more accurate than that ominous error 404.