New version has been released wich hopefully fixes definitively the last day of month bug.
The update procedure is as usual exept that this time the awstats package has also been updated. So just execute this commands :
tar xvfz awstats-6.6.tar.gz -C /home/admispconfig/ispconfig/tools
chmod 755 /home/admispconfig/ispconfig/tools/awstats-6.6/
ln -s /home/admispconfig/ispconfig/tools/awstats-6.6 /home/admispconfig/ispconfig/tools/awstats