I think i'm getting better at understanding this problem.
I'm thinking it's memory related because.
1) if i render a simple cube say dimensions 5000 x 5000 the save button appears. I rendered it 10 times in a row and it worked.
2) I load Victoria 8 fully textured but no hair, try to render the same scene... it works... i have a save button. I save file and repeat the process, the second time it will not work, save button is grayed out.
I have a 1080ti and i pushed the out of core memory to the max. I'm on windows 10 latest release.
not sure if this helps you, but I really would like this to work. if you can't reproduce it, would it be possible just to delete the code that greys out the save button? for some reason it ends up going to that subroutine.
Asus Z97-A / usb 3.1 - Windows 10 - 980 ti - Titan Black - Titan Z - Daz Studio