Static images and CSS not showing (401) but bundled css and js do

I was able to fix it by..

1. Navigating to folder that has css files in IIS
2. Double click "Authentication" icon on "Features View"
3. Right click Anonymous Authentation as shown in the pic and select Edit
4. Select "Application pool identity" option as shown in the pic
5. Restart the website

I could finally figure it out ...

1. Anonymous authentication was enabled (and using default config implying that the user seen by the code is the user IUSR).
2. The files for the web app were not stored locally where IIS is installed, but on some sort of network shared drive ....
3. In that place, IUSR had no read permissions, and could therefore not read the static resources.

I had to change the default "pass-though authentication", specifying that local execution is not done as the "authenticated users" (IUSR in that case).

In the WebApp Basic Settings in IIS, do not use "pass-throuh" authentication, use "Connect as"