Moving config.ini to /Users/<<username>>/Library/Application Support/factorio/config/config.ini fixed the issue.

As in the title, the game will not start on my mac, crashing with "unexpected error" at 10% of the loading screen, after the "loading mods" phase. I have created a config.ini in factorio.app/Contents/config with the following contents:

[graphics]
video-memory-usage=low
graphics-quality=low
and ...