When I (tried to) DL with a mixed grip, my back hurt bad.
When I did some light SLDLs (to below the knee), and turned my head to look at what angle my spine was, I got the same type of pain. (despite the fact that my back was in the same position, i.e. not rotated)
I do rounded back work (45 degree hyper, up to 35 kg/80 lbs x 18), and it feels great. I had some pain when going to flexion before starting this, now I can go into flexion painlessly... (I DL with straight stiff back though)
So, at least for me, it seems any kind of spinal rotation under load (or rotation + flexion even w/o load) leads to incredible pain. Flexion doesn't. Ever.
Is anyone else like this?