Hmm,
I tested again with layouts/ containing the same as themes/minimal/layouts when I fired it up locally and get 404 Not Found as well, unfortunately. Tried emptying the root directory’s layouts dir and still 404 Not Found with your suggestion pasted in.
I must’ve had an older version in the git repo with a different format? but it should be the same as your suggestion now and I can confirm the 404 error is still there.
I can’t see anything reading the files or the directory structure that could be interfering with loading from themes/minimal/layouts/_default/single.html
This is in config.toml, however:
[[menu.main]]
url = “/about.md”
name = “About”
weight = 2