Page not found - %20 in url

The ucfirst() on api is not really relevant, it just means that the router went through the whole set of URI segments and couldn't find a route, with api/session_controller being the last thing it checked.

Since the router can't find your route, I'm guessing that UserName is part of the route it's trying to look up (or part of the controller/method pair), and 'User%20Name', 'User Name', and 'UserName' all require different route definitions (or one well-defined RegEx) to work properly. In most cases, I wouldn't consider spaces or %20 worth the time to support in my application, but if you need to do so, you should be able to run through the application with a debugger to make sure you know exactly what is being passed to the router in this instance, then update your route definition accordingly.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide