Throughout the Democratic primary season there have been allegations of voter fraud in many states. The Democratic National Committee vowed to loo into and investigate these allocations. To date they have done very little to quiet these allegations and in my opinion they are attempting to cover up the fraud which could be costly in the general election.
I think that the evidence we've seen so far (regarding exit polls) is not a particularly overwhelming indicator of fraud, but I think it's a reason to look into the situation more closely. One instance that comes to mind is the video of the Chicago 5% vote audit where it was found that votes were very clearly switched from Bernie to Clinton for whatever reason. In other words, I don't think the evidence is there to conclusively say fraud is happening, but I don't think we should brush it aside either.
On top of everything else this election year has brought, voting regulations have been a huge topic. The system has been called corrupt by all sides of the spectrum and begs to be looked into. I can not say for sure if the DNC willfully ignored election fraud in Chicago, only an investigation will prove it.
Some people will make any claim they can to try and skew the election results or damage a candidate's reputation. These cries are usually unfounded, and just meant to harm a campaign. The votes in Chicago aren't even enough to make a difference anyway, so let's just move on with the election.