Well, I can't say you overdid it providing details about your setup  .

The error is a buffer allocation error, so I'm guessing you're running win10 and have too little swap available to support >= 6 cards? If so, is it possible to increase it to at least >= 48GB?

Another wild guess is that your 6th is a smaller gpu, but you have specified a cn_config that tries to allocate more scratchpads than the gpu can support.