Ive got the exact same problem with the error code, WP015, on my laptop, been like things for quite a while now, can watch live programmes, live BT Sport, but not the on-demand content, tried all the fixes that has been given out, nothing works, tried it again yesterday, same problem, have given up watching it on laptop now, we get a lot of visits from grandchildren who take over the TV, so it was perfect for me to keep watching something, especially the on demand things on BT...... given up in the end, got BT Extra Box instead.
Has anyone got the new app to work? For some reason it doesn't recognise my computer as the same device. Thankfully only one day before I can try again.