Some systems seem to have no problems when running the two together and avast has usually got along well with MBAM but iv seen this detection in the past and lately iv seen it rearing itself again.
It could be due to some of the increased scanning capabilities of avast in the new version but i cant say for sure plus the new version of MBAM may be slightly different in its code as well but it's always recommended to exclude security software from one another for this exact reason, not just avast but all AV's should have exclusions.