can i force it to create a log file or something for you to look at?
the problem is unfortunately not at our side, but on MacOS side (where they repeatedly fail to keep compatibility even with their own latest "inventions").
in this case, it's launchctl utility that fails to manipulate the plist properly (effect of the former -w switch).
Could you do: launchctl unload <path_to_the_avasts_plist> ?
After this, avast shouldn't be started again by launchd (launchd is an intended replacement for old good init/cron daemon suite, root of all evil). When doing the same launchctl unload with '-w' switch, nothing is written back to the plist file (as before 10.6), but is stored elsewhere on disc (/private/var/db/launchd.db/com.apple.launchd/overrides.plist).
So, next version will have to use fourth invocation style (not startup items, not login items, not launchd with modified plist, but plist overrides). Developing for MacOS is "fun"
.
regards,
pc