I can't see how close your shins are to the bar. Could be bar position/setup. Hamstrings are the first thing that give out before the weight starts to move, which means your low back is taking the brunt of the lift.
So either setup is slightly off or hamstrings need work - maybe a combo of both.
The 545 lift in particular you get very rounded. Try and pull more w/ the upper back. Stay tighter up top and grip the bar as hard as possible. Some think you should just let your arms dangle like meat hooks, but that creates a loose back.
Remember also, the bar will not move until your body leverage is at its optimal position for the weight to move. That's why set up is vital.
Strong lifts and keep working hard! Don't forget, that 545 looks more like a max so things will start to appear when you get up that high. The 455 doesn't break down as much.