D5100, SB700 and SB910
Write a snail mail letter to Nikon. You'd have to get in contact with them to ask who to address it to. I don't like your chances. It is getting to the point where it's not worth repairing Nikon gear and I suspect that this is exactly how Nikon wants it. You'll have to decide if the small chance of getting resolution is worth the effort.
If you can find a setup that works for you with one of your other flashes as master, I'd go with that. Costs you nothing. Least stress. Otherwise sell the SB-700, disclosing the problem and put that towards a new flash.
My forum postings reflect my own opinions and not those of my employer. I'm not employed in the photo business.
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