For the question raised in the priority support forum on this subject , re-running the squirrelmail-configure script did not work. In my case, the open_basedir problem had to do with the spool directory: /var/spool/squirrelmail/attach/ . So, what I did was modify the /etc/apache2/conf-enabled/squirrelmail.conf, and added that path to the open_basedir path list.
