I too can now not use BT in my BMW after 5.1.1 update.
Reset the device to factory settings.... Did not work.
Updated car software... Did not work.
Smart lock disabled... Did not work.
I am so annoyed. It seems that lollipop 5.1.1 has many issues, which probably means that this one won't be sorted any time soon.
Has anyone got any more suggestions?
I have sent a very snotty email to Samsung basically saying:
"I have a phone that now only lasts 20 hours on it's battery, gets hot, is slowing down after a few months and now doesn't play music in my car! It will soon be useless and I have 18 months left of my contract!
But at least it looks pretty hey..."