I keep getting this warning (see attached image). It has been happening every half hour for days, and it is very annoying, especially since I have not visited palettegen.com.
Is there any other solution besides uninstalling Avast, considering that SupportMail is not working?
The instructions are there for investigation only (to try and determine the cause) if nothing else to confirm or deny if one is the cause.
It isn’t there for you do for eternity.
Try running a scan with
Norton Power Eraser and Sophos Scan & Clean
You don’t have to install them but run them and they will clean you from any malware, cookie trackers, etc… It’s a second opinion scan that is always better to do with any AV.
It makes no sense to me to disable extensions that I’ve been using for years. 
The problem is with Avast. I guess I should disable Avast if I don’t find another solution.
Is there a way to contact the Avast support team?
did you clear evrything in your browser?
In all major Internet browsers (e.g., Chrome, Edge, Firefox, IE, Opera), pressing Ctrl + Shift + Delete opens the Clear browsing data window to quickly clear private data and history
did you disable all extensions in your browser?
I did everything listed. Nothing helps, I get the same notification from Avast again.
Uninstalling Avast wouldn’t stop the attempted connections to the site, you just wouldn’t see the alerts and you could be at risk.
As you weren’t intending to visit the site:
Start by clearing your browser cache and cookies, including 3rd party cookies and restart your browser.
If that resolves it you should be good to go.
If it doesn’t try running your browser with add-ons disabled.
If that resolves it, have you added or updated any add-ons ?
If so try disabling that add-on - and restart and try again.
If still not resolved and you are using Chrome based browser - try this suggest by Avast Team member ‘lukor’
What about page notifications? Here: chrome://settings/content/notifications ? Do you have all cleared?