1_ Don’t let Avast take any automatic action. If Avast asks you what to do with that “infection” and you can’t skip it, then you would need to add a temporary exclusion / exception.

2_ You need to provide more info. What exactly is being reported as infected? What infection / malware is detected? Is this being changed from the original (compare it with checksum)? Can you scan in the original system the exact same items that are “detected” in the backup? Have you checked / scanned the backup software itself?

3_ The antivirus definitions are the same across versions, so I don’t think there is any difference in this case.