In case any other Mac users have this issue: Publii will fail without any error message if the username is different and you simply copy the directory from Documents or sync it with iCloud . Turns out the user directory path is hard-coded in the config and site data. That makes no sense since storing the Publii folder in iCloud Documents makes it portable. Or should be. By storing the full user directory path in Publii’s config, that only works as long as the username is the same.
To fix it, run a search and replace and replace ‘Users/oldusername’ with ‘Users/newusername’. The easiest way to do it is to download TextMate (free) and use its Find-Replace feature.