The problem is that the latest Open OS version is based on the stock OS version 1.4.2 which is not the latest version (1.5.1) you probably updated to. So, you might have a look into this thread. There are two options for you now: either you wait another couple of days/weeks until the release of the next Open OS version (which will most likely happen by the end of this month) or you try flashing the image via fastboot (see linked thread).