You could try putting the dll back and see if setting the version in Web.config to match it fixes it. Its declared in the assemblies section of Web.config. You could also try just commenting that out. As far as I know the only thing that uses that preview dll is the MyPage feature. The preview dll allows the webpart drag and drop to work in Firefox and Safari.
It looks like I may have updated the dll but not the declaration in Web.config
Hope it helps,