Fish virus are often species specific so it would make sense that the problem could be viral. Additionally they have a long long latency period where nothing happens for about 3 months and out of nowhere the fish dies. There's absolutely no way to cure fish viruses unfortunately and it's almost impossible to diagnose without someone doing some protein assays/microscopy so there's no way to know without extensive and pricey testing.
IMO I'd leave the tank alone and not add any more angels for a few months. Hopefully the viral particles would have degraded and it will be safe to add a new one. I'd also add one from a fish store that has higher quality stock.