It's possible that one of the strobes is not performing properly. If you swap the strobes, do the symptoms reverse? A month ago, one of the members of our photo club gave a talk on strobe positioning. He described testing the strobes to make sure they were delivering the full guide number power. He'd had experiences where brand new strobes were not. Using a light meter, set strobe on full power 3 feet from a wall. Fire the strobe and measure the reflected light. With a GN of 24 divided by 3 feet, should read f/8. I've probably over simplified it, as I don't have a light meter. But hopefully, you get the idea.
You don't mention what your shutter speed is set to. Back in 2015 I had a problem using YS-D1's in TTL mode, getting the black bar. I received a response from Sea & Sea Support stating that the YS-D1, and YS-D2, is not designed to sync above 1/200th.