I discovered today when setting up my brother in laws Amazon Fire Stick that if the users password has the £ symbol in it you receive a message that the Username/Password is invalid.
Changing the users password to not include the £ symbol allows the user access. I reset the users password before changing it to not include the £ so definitely knew I was entering the correct password. Other symbols seem to be ok, tested with ! and $
This only seems to be an issue on the Amazon device as the user could log in via a web browser and the Android application.
Just thought I'd let you know, as it was a pain.