I have a relatively old nVidia card (GeForce GTX 770) but one that’s recent enough that it should be supported by the latest driver. However I had issues where if I stopped using darktable for 20-30 seconds and then went back it threw an error and reverted to CPU. I ended up having to go back to the legacy driver (340xx series) and now I don’t have any problems. So if possible it’s worth trying one of the older 340xx or 390xx driver series and see if that resolves the problem.
Try rerunning from the command line with -d opencl and see what gets output.