To fix this problem Try disabling the antivirus scanner on the content filter.
Click on the Proxy Tab, Click Content Filter subtab, edit the default profile and deselect the checkbox for activate antivirus scanner.
I also ran the following commands:
cd /usr/share/clamav/
rm -Rf clamav-*
rm main.cvd
rm daily.cvd
rm safebrowsing.cvd
curl -O http://db.local.clamav.net/bytecode.cvd
freshclam
restartclamav
/etc/init.d/clamd restart
restarthavp.py
Any pages users were receiving the error message on should load correctly after refreshing the page.