DavidR
2
The problem with two AVs on one system is the likelihood of detections of each others virus definitions.
This from the path given is in that area definition updates.
When a third party AV (Avast in this case) is installed, windows should shutdown defender to avoid conflict. Whilst windows update will still be trying to keep the virus definitions up to date, which could also cause issues unless these updates/definitions are encrypted they could be a false positive detection.
A search for one of the two files https://uk.search.yahoo.com/search?p=nisfull.vdm should give you an idea of what I have been saying.