Looks as if your system uses the wrong amavisd.conf file. Centos has a big problem, depending on the repositorys that you use, there are different amavisd packages and one uses /etc/amavisd.conf and the other one /etc/amavisd/amavisd.conf as configuration file. In your case you seem to have installed a package that uses /etc/amavisd/amavisd.conf but ISPConfig expects that the file /etc/amavisd.conf is used. So if you have thes etwo amavisd.conf files on your system, run the commands:
mv /etc/amavisd/amavisd.conf /etc/amavisd/amavisd.conf.bak
cp -pfr /etc/amavisd.conf /etc/amavisd/amavisd.conf
and then restart amavisd.