I created a web tab to display an external web application. This is what happens in Classic and Lightning:
Classic - The web tab loads the web application and I can even login into the application. CSS, and other resources gets loaded automatically. No issues here.
Lightning - The application doesn't load and I get the following error:
File not found The resource that you have requested was not found. Please check the link and try again.
I decided to whitelist CORS with the web application URL but the result is the same.
Have you come across this and if so, how to load the external application inside Lightning experience using Web Tabs?