Set up is key...getting everything right just before a pull was a big difference maker for me.
Heavy sumos might be safer for your low-back but they aren't necessarily training your weakness for the conventional. Some ideas are a slight conventional reset and laser focus on form or finding an assistance move that is low-back-form intensive, like GMs, RDLs, or whatever works for you. Doesn't have to be heavy, just has to target the right area.
That said, I'd still bet there is some issue of hip mobility or hamstring strength at play. Could also be weaker hip flexors. Sometimes, the weak spot is not where you think it is.
If you think the cube-style rotation is good then keep it. If it's too much of one aspect or not enough of another consider tweaking it.
There are other increments between 385 and 405/415....consider making smaller jumps. The CNS success with smaller jumps will help you open up heavier lifts and you're less likely to overshoot and pull a muscle, etc.
Or, maybe it was just a crappy day!
That's my 2-cents...I wouldn't be surprised to get back some change.