I recommend taking everything off the D6000 but one external display (try each external display as the lone display) and then add each other device back to the D6000 one by one (in every combination) until you see the error messages. If it's just a specific device combination that causes the problem, you should be able to pin it down in a reasonable amount of time. You may have to obtain a second USB hub (as I have) in order to follow the suggested procedure.
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