Hi ansentry,
I'm not an expert, more a tinkerer

- but here is some food for thought...
the settings for clam scan are done from the server-manager (Configuration / Anti-virus (ClamAV) - you have the choice of daily or weekly scans or never - so its odd that your errors are coming through every few days. Try setting it to never and then save, then back to daily (or weekly if you prefer). That should rewrite the cron.d entry. Perhaps if your pc is a bit under spec'd the daily scan may be taking longer than a day and attempting to start again before finished.
The log details are sent here according to my copy of /sbin/e-smith/smeserver-clamscan --> /var/log/clamd/smeserver-clamscan.log - do you have a copy of this file? Does it appear in good health? Does it have any further information about the error? Note I have just checked mine and I also have a clamscan.log file in that directory - which appears to have more the details i see in my weekly clam scan email.
Also you could try running from the console prompt to force the scan and check if the logging error comes through every time.
/sbin/e-smith/smeserver-clamscan
Word of warning my scan takes almost 800mins! And is quite resource hungry! But if yours is kicking out an error then that should be pretty quick in sending the email and updating the log files.
Good luck! Cheers, Shell