The corruption is that they repeatedly and publicly claimed that they were neutral in the primary process and were clearly not, acting in favor of their preferred candidate as many had said they were. Bernie supporters were dismissed as fringe lunatics believing in a conspiracy theory for reasoning that the DNC was coordinating against them.

No individual is truly neutral. The concern is whether the organization performed any acts of favoritism.

I don't have an answer, but that seems like the only possibly-important question.

There were some moderately-questionable emails about where donor checks were being routed. It would take someone with more knowledge of campaign finance than me to say if it was illegal or unethical, but the recipient of the email basically answered "don't send this stuff over text, you idiot".