When it is a Net bad thing
Sorry for the pun, but this is a classic case of a solution to a problem that never existed: in fact the solution is causing problems. It was a terrible mistake to implement electronic voting, especially if it's done with no backup paper trail. It is even more asinine to make these things accessible via the internet—we are talking about mighty temptation put before bad actors with the resources of a nation state. It's a recipe for catastrophe, and that catastrophe may even have happened already—look at how a few delicately chosen small majorities gave Trump the presidency despite a shortfall of <3m votes.
Someone's already pointed out that a well-designed physical system can do the job securely and with admirable respectability, like that of Britain: with enough eyes seeing, literally watching what's happening, and people covering the chain of custody of ballot boxes, it is a traditional, old, antiquated and highly effective system. There is simply no good reason to add technology, especially when that clearly introduces a swarm of horrid vulnerabilities.
That the world's foremost and wealthiest power, which claims to be a democracy, cannot simply fix this, beggars belief.