User account no longer in ‘admin’, how to recover?
After starting to Single User Mode enter:

/sbin/fsck -fy
/sbin/mount -uw /


Launch opendirectoryd:

launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist


Then enter (replacing username with the real user name):

dscl . -append /Groups/admin GroupMembership username


You will get an error "...com.apple.DirectoryServicesLocal.plist doesn't exit anymore ..." - ignore it! Your user should still get promoted to an admin user.

Enter exit to boot to the GUI.