Mbox 2 does not support sample rates of 49 kHz or above due to the bandwidth limitations of USB 1.1 and the USB Controller. This is only an issue when syncing to an external S/PDIF clock that is pulled up 2% or more above nominal 48 kHz sample rate.
Taken from the "Read Me" for Pro Tools LE 7.0 for Macintosh and Windows. (Item # 63141).