You'll have to completely purge clamav and it's definitions, then reinstall it.
Do something like:
apt-get remove --purge clamav* libclamav6
Delete everything from /var/lib/clamav if anything is there.
apt-get install clamav-daemon libclamav6
Make sure to configure clamav according to section 7 of the guide.
Let me know how you make out.
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