I'm in the process of switching from my mirror-less Panasonic GX85 setup with Inon S-2000 strobes to a D810 in a Sea and Sea housing with optical converter and 2 YS-D2 strobes. So, lots of learning to be had.
My problem strobe is not working when set to TTL and optical converter set to TTL. The strobe is flashing but is out of sync with the camera so the strobe light is not captured by the camera. However, the problem strobe works fine when set to manual and the optical converter set to manual. I have tested the problem strobe with both fiber optic cables and the cable does not seem to be the issue. It seems that something is wrong with the TTL settings internally to the strobe.
The other strobe that I have works great and is in sync with the camera in both TTL and manual modes. I have set the custom mode to A for both strobes.