Here are a couple of things I'm wondering about, based on this conversation so far:
1) Has anyone looked in avast! settings: confirmations: "Message when results were successfully processed"? I'd make sure this box is checked, as well as the one below it, "Message when error occurred during results processing".
2) For the option avast! settings: common: "Show results of Explorer Extension", try UNchecking it, restart, REcheck it, restart, and see if that solves it. Sometimes, with some programs (not necessarily avast), flags (checkboxes) get out of sync between the UI and the config files or registry entries. It's worth a try to toggle it.
Good luck. I hope you find a resolution.