You are getting the rootkit warning because the SVC MBAM SwissArmy is a low level driver used by Malwarebytes, as mentioned before you can make exclusion between avast and MBAM or just exclude that single path.

It would also be beneficial to all avast users if you could forward that detection to avast as a false positive, I don’t know why you are receiving the warning though as many have MBAM installed here and there have been no other reports that I know of ???