Finally I managed to find it out:
OnUpdateExecute is not accepting commands with multiple arguments. I arrived to this conclusion after creating the script /home/admispconfig/ispconfig/tools/clamav/bin/OnUpdateFreshclam, which just changes the permissions:
chmod -R 755 /home/admispconfig/ispconfig/tools/clamav/share/clamav
then I chmoded it +x and changed OnUpdateExecute in the file /home/admispconfig/ispconfig/tools/clamav/etc/freshclam.conf from:
OnUpdateExecute 'chmod -R 755 /home/admispconfig/ispconfig/tools/clamav/share/clamav'
Now it works perfectly, I have tested it with real freshclam updates.
I think this should be changed in future ISPConfig releases.
I'd like to have some feedback about this issue in your machines, and also about the default permissions for /home/admispconfig/ispconfig/tools/tcpserver/ispconfig_tcpserver and its relation with spamassassin.