To be honest I have not heard of a story where an AV update went wrong and the system could not defrag the drive. Alright, maybe unable to shut down that is plausible.

Online Armor is not bad because it is well respected and earned good marks on the Matousec firewall test.

It is not the firewall that is you the alert about AVG's updates, rather it is the HIPS (Hosts Intrusion Prevention) that is alerting you.