Faced the same issue. Trackpad works while system boots up but becomes unusable immediately after logging into the system. For some weird reason this helped: "System Preferences" -> "Accessibility" -> "Pointer Control" -> uncheck "Ignore built-in trackpad when mouse or wireless trackpad is present". Never would expect this to work because I have not used any of those external devices with my MBP. What is more surprising is that it continues working even when you turn this option back again.