I did what you suggested: I deleted all the entries I could find in Comodo Firewall (including Defense+)that were related to Avast. I then rebooted my PC… and no, it didn’t solve the problem. I am still getting a popup from Avast saying that the last attempt to update the definitions was unsuccessfull. In that same popup, it prompts the user to “update now”, which I did… again without success.

However, here is something interesting: Using Rollback RX, I rolled back to the snapshot I took prior to updating Avast from v. 6.0.1289 to v. 6.0.1397. Guess what… Avast had no problem to update the virus definitions! So, it seems obvious to me that something went wrong when updating to version 6.0.1397 (whether it’s a bug in this version or something else I can’t figure).