First, be sure to get rid of all remnants of MSE…!!
https://www.avast.com/faq.php?article=AVKB11#artTitle

Then;

  1. Download Avast Free Antivirus: https://files.avast.com/iavs9x/avast_free_antivirus_setup_offline.exe
  2. Follow instructions: https://www.avast.com/uninstall-utility (Run this tool for all prior installed Avast versions…!!)
  3. Reinstall Avast with the downloaded installer from point 1.
  4. Reboot.

PS: Regarding Avast & MBAM, see: https://forums.malwarebytes.com/topic/200324-exclusions-for-avast-users/