Um, i just realised that there is a good chance that because freshclam switches to another user other than root, it has no permissions to access /var/log so you may either create the file and add 'admispconfig' as the file owner or change the path to another writeable path allowed by the 'admispconfig' user.
tested as below :
exec('/home/admispconfig/ispconfig/tools/clamav/bin/freshclam --log='.$conf['logfreshclam'].' 2>&1', $output, $retval);
Output I got.... which is what you probably have too...
root@ispc:~ # php clamavupdate.php
 => ERROR: Problem with internal logger (--log=/var/log/clamav-fresh-update.log).
 => ERROR: Can't open /var/log/clamav-fresh-update.log in append mode (check permissions!).
so basically do a :
chown admispconfig /var/log/clamav-fresh-update.log
or change the path in the file....
give that a try.. damn coding.. always have to delete all and start again to make sure bugs/problems are found.....