I was using tinkercad and I did test it at 1mm height before and same issue. S3D refused to print this object no matter what I do. I'll just make it out of rectangles instead, rather annoying since Cura is A OK with it and S3D has a fit!
EDIT: Ok that solved it, seems tinkercad was causing errors in using a large primitive and cutting out the inside like I did, still strange how Cura was ok with it, anyway solved for now. Cheers.