After upgrading ISPConfig3 you might see something like this in your log files... (I see it in our ProxMox/OpenVz servers.) After a while your server will crash because PHP eats all the memory. You can fix this by running the /usr/sbin/squirrelmail-configure again.
Upgrading the Debian. apt-get dist-upgrade Then "updating" the ISPConfig 3.0.1.3 to ISPConfig 3.0.1.3 After that it's Squirrel config.
I've noticed this in my logs as well, but I didn't think it was filling my memory up. So far the server hasn't crashed yet because of it.
There is a bug in maildirmake which may cause this under certain circumstances. ISPCOnfig 3.0.1.4 will contain a fix for this.