If you look in the log, do you have this error?

SuperFly: OptiX CUDA error CUDA_ERROR_ILLEGAL_ADDRESS in cuStreamSynchronize(cuda_stream[thread_index]), line 690

Symptoms as you describe. The render stops partway through, the remainder just goes black. If you do another render, it fails immediately and is all black. The original frame that caused it will always generate the same error; subsequent frames or other scenes may work if you restart Poser.

I was getting that a lot but it has been reduced in 12.500. Before, any frame that caused the error would never work with GPU rendering (Optix or not). Starting in 12.500, those frames now work in Superfly as long as I don't use Optix. That's a big improvement because rendering without Optix only takes twice as long, whereas CPU rendering is more like 10x. (I was originally having to reboot, too, but now restarting Poser is enough.) There are also fewer scenes that cause the error.

If you aren't already on 12.500, definitely update to that version.

Do report it though. They need example scenes that generate the error in order to troubleshoot it. It's very hard to fix an error if you can't cause it to occur on the test machines.