I had this exact problem after upgrading my old MacBook Pro from High Sierra to Mojave (I still use it for 32-bit apps so don't want to go to Catalina). I tried every suggestion I could find by googling with no success, until I came across this:
https://whitehatmac.com/macos-bugs-are-causing-kext-failures. This blog post was written by a guy who works for Malwarebytes, and the suggested workaround for a macOS bug immediately solved the problem, allowing Avast to work normally. Furthermore, the post provided a very clear explanation of the issue.
It's a little disappointing that I was only able to fix this problem by taking advice from a competing company, after Avast itself basically said "there's nothing we can do."