Little typo I'd say in the "plugins/Diagnostics/Diagnostic/WriteAccessCheck.php" file on a fresh install.
$directoriesToCheck = 'config/';
$directoriesToCheck = '/config/';
Or the checked path will be for example : /var/www/piwikconfig
Sorry for duplicate, #8439 title was not very explicit, so I did not find it searching for piwikconfig keyword
I have updated the issue title so that it's more explicit.