I think this is more a Dreamweaver and/or Extension Manager issue that Webassist. I have run into problems with other extensions not uninstalling completely.
Have you uninstalled all of your WA extensions? memento is included in all of them I believe.
If you still have some WA extensions, try re-installing one of them, and see if that fixes it.
Or perhaps re-install one, and then uninstall it to see if it uninstalls is completely that time.
The other thing you can do is delete the WinFileCache (or MacCache) in your configuration folder, as the PDF mentioned above suggested.
I find that often cures errors such as that.
