Hello,
Not sure if this is new to you but
213.96.25.30 - - [05/Mar/2009:19:22:14 +0100] "POST /roundcube/bin/html2text.php HTTP/1.0" 406
and as a result a non-empty directory /tmp/guestbook.ntr/ is created and a file /tmp/guestbook.php (which then causes issues with the operating system).
This html2text.php file has been used by an attacker on my system (at least I think so). I have removed roundcube from my system and since then I have had no trouble, although they have been scanning for this file as I read from the logs.
Yours,