Asyn pointed me here from my previous thread - https://forum.avast.com/index.php?topic=201147.msg1388159
Essentially all of our Mac's, mixture of 10.11.6 and 10.12.4, have updated to 12.6 32612c2abb90 and doing a Full scan causes an internal error.
I've attached a system log to the other thread but since then i've managed to do a little testing and reinstall 12.5 (luckily we had the installer lying around) on a few Mac's - so far we haven't had an issue with doing Full Scans, it appears to work correctly.
Updating to 12.6, from the Preferences panel, causes the Avast app to 'Force Quit' and then we experience the internal error issue again. I did however spot a lot of unusual messages within the Update log during the updating process, which i captured on a screen-record, and have attached a few screen grabs.
Any help will be appreciated!