an OS X server got unhappy and seemingly did hang on bootup on the gray screen with the rotating ‘progress’ indicating.
Booting it with text mode enabled (holding command V) revealed that the machine would loop with the following error message snippets:
posix_spawnp ... /usr/sbin/mDNSresponder ... no such file or directory
ls -lsd /
0 drwxrwxr-t 43 root admin 1530 Dec 20 03:57 /
Booting into single mode (command -s) and then doing the suggested fsck and mount -uw / revealed that the permissions actually were set to:
drwwrx---
Meaning that others have not even read permissions. This makes the mDNSresponder seemingly unhappy. I would guess that it quickly gives away its root privileges after launch for security reason. Running what the internet suggested
chmod 775 /
did fix the issue.