That's normal. That's where horde writes it to on a couple of dedicated servers. It's either just putting it there because of the php's default location for temporary files, or that's where horde's programmers wrote it to go.
Either way, this is something that would have to be addressed by Horde's developers if they agree it's something that's urgent. Personally if you're giving SSH access out, etc I would not be worried about the horde log file.
