I'm not sure whether this is an issue with the main BT Consumer Site (My BT) or the app itself but I think there is definitely an issue somewhere.
I use a Password Manager, to create separate log-ons for all websites which require them, for extra security I generally choose *very* long passwords. The main BT site quite happily accepted and worked with such a long password. However BT Sport on Android failed with the ID004 Error. This occurred in the middle of the night on two different devices. Resetting the password down to a much smaller value <20 characters allowed the login in to succeed.
I'm guessing but ...
The Main Website is good with long passwords or is it silently truncating them? (form code accepts <=50)
BT Sport app is failing and should be supporting passwords up to 50 characters long.
Also could the usability of the sign up pagebe improved if there were some hint text/validators there to indicate what the maximum password size and expanded to state which non-alpahumeric characters are acceptable?