Actually, I'm not sure why it happens on my VPS. The situation is, after the VPS is rebooted, mime_drivers.php is changed to root:cpanelhorde with permission 640, so of course, I can not access address book in Horde. I find two ways to fix it, one is changing the group to cpanel-horde, and the other is changing the permission to 644. However, after a reboot, the group and permission are changed back again automatically. I have to change them correct manually every time. Does anyone have the same experience with me?