I know from past conversation that you are Mac user, but in a quick look I don’t see what version you are using. Usually what Walker suggested - reinstalling QTR (not P-T) - does solve this problem when it pops up, but apparently not always. Elsewhere you said that you haven’t printed in several months. Any chance that between then and now you have upgraded the OS? What version of Mac OS are you using?
I think what is happening is that QTR isn’t actually using the quad even though you may see it in the dialog. If I am right about that it is actually not using any quad at all, hence the nearly all black negative.
I think the next step, if you haven’t done it already, is to go to the Library folder where quads are put when you run the install command, and delete whatever is there, then reinstall them from the Curves folder for your printer.
The path to the Library folder is: MacHD>Library>Printers>QTR>quadtone>Quad1430-PiezoDN.
The path to the PiezoDN folder is: Applications>Piezography>Curves>1430-PiezoDN.
Hope this helps,
PS - One more thought - just a nitpick about terminology: Your prints are not overexposed. They are properly exposed. (Unless your chemistry and/or exposure light has changed too, then they might be overexposed, but you didn’t mention that so I assume it’s not the case.) It’s the negative that is the problem here. Your prints from very dark negatives must be very light, which in terms of negative/positive print systems could be termed underexposed but not overexposed. Overexposure lightens the image in direct positive systems like slides/transparencies or Cibachrome prints.