I am having the same problem here:

Avast has completely disabled Windows Defender, nothing will enable it (neither going to “Services”, nor command line, nor the Windows Defender UI, nothing) which means it can’t update itself, which means I have a 150MB file trying to download and install itself via Windows Update everytime a new definition update for Defender is pushed out, which is everyday!

Do I have to uninstall and re-install Avast everytime a new definition update rolls out? (aka everyday)

Anyone has managed to solve this?

PS: For whatever is worth, the Windows Defender service in “Services” is set to automatically start (shouldn’t avast set it to not auto start?) but can’t even be started manually, it fails to do so with a cryptic “signature is not valid”.