I too experienced the problems with repeated error messages, and tried all the remedies here having had a stable, working fine Dreamweaver CS6 (Win 7, 64 bit) with installed extensions of Site Assist Pro, CSS Menus, CSS Button, CSS Sculptor, Palette Picker. Nothing eradicated the problem error messages.
In frustration, today I took another crack at it and made things worse (more than the one error I'd previously got it down to). I've now tried to eradicate all WAssist extensions and reference to them: all WA extensions are removed (not just disabled) through Extension Manager (that didn't cure things), I've searched the registry for Webassist and deleted references there, I've found all the files and directories in Explorer that I can and deleted these, and deleted the DW cache/preferences.
Now I have a different error - the old ones for Memento and RSS feed and 'can't find' errors for line 12 and .js files are all gone, but I now have two relating to floaters that show on starting up Dreamweaver:
Unable to find the file WA_Memento.htm in the Configuration/Floaters directory
Unable to find the file WA_SiteToolsProgress.htm in the Configuration/Floaters directory
No doubt because in my frustration I deleted these files. Putting them back introduces different errors, so what I need to track down is what and where these are being called from. Where is the command that is asking for these files? It must be buried in something I can't find ...
Then, perhaps, I can put back my paid-for extensions that used to work just fine before Free Tools (which replaced some older extensions that worked just fine).
Thanks for help with this ...