Because it's not done correctly?
That thread should fix pretty much any of these issues if you read it through carefully and follow what it suggests.
If the game tells it's the wrong password, it is. Now keep in mind that it was mentioned in the thread that it's capital sensitive, and that you cannot use your numpad when you write numbers, just using the numbers over the letters.
FH just uses the English keyboard layout, so if you got anything else set to it, things like symbols or even letters work differently within the game than on the desktop.
What exactly is their username? Would help to know if the username could be a problem or not, and if it isn't it would work if you reset/changed your password, otherwise I sense you aren't doing it correctly.