It’s good it never was a problem for you, but there is no question MBAM is the reason the installed failed. Perhaps you’re only looking at it from the angle of MBAM looking at Avast as a virus. Could simply be something in MBAM that stops programs from installing a key to the registry, the operating system, or something to that effect. But, I haven’t a clue because MBAM never appeared to be in the way until I disabled it on start-up.