When i go to Edit mode and look into the UV, if i zoom i see that at the location of the problem, the UV has a space :
That look strange to me, as i don’t see why the UV is not continuous there and there’s a separation.
Did you re-unwrap this model yourself or is it the direct unwrap from Makehuman ? anyways, in my opinion this is not really good at all to have this separation, it’s not good practice for this kind of unwrapped character.
It’s very odd that it is impossible to paint over that section, i think there’s some kind of bug with the UV there, if this is the default Makehuman UV i guess they have a bug in their low poly model unwrap method, or they’re using some kind of transfer UV between high and low poly that just fail badly at the center of the body.
Anyways, i fixed the UV map with making the seams from the UV, pinning the border UV islands vertices so the UV islands would keep the shape they had, so you would not have odd result when using the original makehuman textures, and i re-unwrapping the body correctly to avoid that bizarre middle separation.
I uploaded the fixed blend on pasteall.
You will be able to paint without the “seams” anymore.
In Texture Painting mode, i recommend you increase the bleed to 4 or 5 instead of the default 2 if you want to paint
Maybe you should report this model as a bug on the bug tracker, as even if the unwrap seems faulty to me, the fact it’s impossible to paint over that odd separation is very strange.