I had the same problem and I performed a couple of factory resets trying different ways of configuring the exchange activesync before figuring out what is going on. What finally worked for me is the following.
When I was entering the necessary fields in the configuration screen (email, server address, user name, password etc.) using the onscreen keyboard, at the end I was pressing the little key at the bottom left of the virtual keyboard to remove it from the screen and then I was pressing the button “next” and I was always getting the message ”authentication failed please check username and/or password”.
When I tried the same thing (filling the fields in the configuration screen) without pressing this little key that removes the virtual keyboard, everything worked perfectly. I did not believe in my eyes and I decided to make again a factory reset and try it again. It worked again….
Hope that this helps