Then, ZA is causing the problem.
The reason that a full install of Avast works is because it likely overwrites all it's files that are left, and only then will ZA recognize the changes.
Any attempt to uninstall ZA without also using it's removal tool and then rebooting is not sufficient. Even then, there can be ZA files left behind that must be removed manually.
I used ZA a few years ago when it was light and not intrusive. I cannot remember what version caused me to remove it, but I was using AVG along with it. Switched firewalls, and things worked fine for another year or two. AVG became bloated and sometimes required a reboot for a simple definitions update, and started causing problems, and it was replaced by Avast Home v4.x.x.