Recently one of our 1500Ds encountered the same issue, got stuck after RAM activation and never started CPU initialization. A strange thing is it comes up normally when we power-cycle it. Then again it gets stuck when we reboot it by a command.
Since 1500Ds support "exe sensor [list | detail]" command, we checked the power supplies and power input were fine.
The first RMA replacement was defective and even didn't boot up at all when we turned on the power. So we had to go through RMA of RMAed device. We're currently running with the 2nd RMA replacement.
Those RMA replacement device stock is mostly refurbished ones after somebody else RMAed it. Like many others, FTNT's quality control for refurbished devices might not be perfect and this situation seems to happen time to time with RMA replacements.