Transparent background, one layer, no selections, tried changing all settings (incl. color), but to no avail. Paint.net simply does not render lines anymore. Not sure what changed in the most recent patch, but I hope to find a fix. For the time being, I'm just going to backdate it.
Computer stats:
64-bit Windows 10
intel core i7- 3630-QM @ 2.40 Ghz
8.00G installed RAM
Nvidia GeForce GT 635M
Running DirectX 12, though some features are limited by my graphics card's capabilities.