Bug: device lock code not recognized

Since the late update, the device lock code is sometimes not recognized. I have to reboot the phone to let it work. Any hint? Edit 30 Jan. 2015: happened again two times since my first message, not clear how to reproduced this, though. Edit 11 Feb. 2015: happened once again this morning, phone feels slightly warm. After reboot, Sysmon shows CPU 50%. Edit 13 Feb. 2015: happened once again this morning and it seems it's not related to the CPU load, almost 0% CPU load this time.