If you don’t mind about the current configuration of Avast, you may try resetting to “factory defaults”.

Avast main GUI → Settings → troubleshooting → Restore Factory Defaults → check both checkboxes > press the “Reset Now” button.

Save Avast new options (and do not change them now that you have reset them), and reboot (for better troubleshooting).

I’m not sure if resetting the settings by this method can change some “avast.ini” setting related to the tray or the display of Avast, but this is the only procedure I can think about. If this doesn’t work, only a full clean reinstall “might” (using the Clear Removal Utility under Windows Safe Mode).

If neither a reset nor a clean install help, then the problem “might” be somewhere else in your system (let’s hope not).