Ok I figured out what was causing the issue. I had some support material (tree Manual) on one of the models. Not a huge amount, only a diameter of 60mm (with a gap of around 30mm diameter in the middle with no support), and only around 10mm high.
That was apparently it. I switched that to ordinary support (still manual) and it got accepted straight away.
What a stupid problem MakerWorld imposes on itself.