Google modified the handling of the Bluetooth "Service Discovery" database (again) and now some cars can no longer be paired with phones using LineageOS 14. Strange enough everything is fine with the original Android 7, e.g. with a Nexus 5X. See here for a list of affected cars.
I'll try to add another hack to the Bluetooth driver to fix this problem, but it may take some time.
One user notified me that he managed to pair his Mercedes SAP V2 adapter with his phone by initiating the pairing from the phone. He touched the "MB-Bluetooth" device in Android Bluetooth settings, while the adapter was (unsuccessfully) searching for the phone. Maybe it works for others as well.