If the Error 404: Not Found message appears during an update, it will be necessary to do it manually:
cd /tmp
# for beta version
wget http://www.ispadmin.cz/update/update_ispadmin_ion_beta_AddHashHere.tgz
# for stable version
wget http://www.ispadmin.cz/update/update_ispadmin_ion_AddHashHere.tgz
tar zxfv DownloadedFile.tgz
cd ispadmin_update
./update.pl