After a few hours continuous playing, the game randomly crashed directly to the main launcher screen, with a java exception message. Since then, the launcher is taking a very long time to start, greater than 30 seconds, even after a machine reboot, so I guess something has become corrupted.  At the time I was not doing anything clever or tricky, just jumped off of a village house onto the ground and then started walking.  Once I managed to get the game running again, it crashed the same way after only 10 seconds of play.