He lost his voice at the end of one of their recent lives - they weren't able to do the encores. He was kind of mortified about it, and apologized to the fans that attended on Twitter. Kai did a post about it on his blog and authorized fans to share it with people not in Heresy - he assured everyone that Ruki was going to be okay. Ruki came back for their next live and was fine. Most likely, it was stress and overwork, given the schedule they've had lately.