If you have an Acronis Active Protection warning that looks like this:
C:\Program Files\Avast Software\Avast\AvastSvc.exe
Affected file: NAME OF YOUR ACRONIS BACKUP FILE
If you have Avast software installed, this is a legit warning message from Acronis and you need to select Allow
Normally it's an update to your Avast security software that has caused this warning from Acronis.