You should follow this set of supported directions as a first step to get it started again:
ClamAV - Reset Defs DB - Zimbra :: Wiki
You can probably ignore the .93 version warning as that will get updated when you upgrade to 5.0.18 (profile has you at 4.5.10). (There is an unsupported guide if you really need to:
ClamAV - Updating Version - Zimbra :: Wiki From time to time there's severe bugs in the version itself, such as:
Techworld.com - Open-source email scanner hit by exploit so read the recent .94 & .95 release notes for corrected issues, but most are comfortable with just up-to-date defs.)