There was an article on this in the Times today - a reputable source, in my opinion. In it, they said the following:
"Is voter fraud a big problem in the US?
There is no empirical evidence that voter fraud occurs frequently enough to affect American elections. A 2017 study by the Brennan Centre for Justice found that the rate of fraud is les than 0.0009%.
Have there been any ballot problems this year?
A few, owing to the surge in demand for postal votes as a result of the pandemic. Virginia sent out 500,000 absentee ballot application forms with incorrect return addresses before correcting the mistake. In Franklin county Ohio, and in New York, thousands of erroneous ballots were sent out, before being resent with safeguards in place."
This does not sound like evidence of widespread fraud to me.
Interestingly, the only fraudulent activity I read about before polling day was the fake deposit boxes for postal votes, that were placed in some communities by the Republicans - presumably hoping that voters would post their ballots in there, instead of in the proper place, so they wouldn't get submitted and counted.