I’m still in the stage of migrating to SharePoint 2013. As a preparatory step, I uninstalled SPD 2010 and installed SPD2013 as SharePoint 2013 requires SPD 2013. At some point in the migration, I had to update a SPD2010 workflow. Guess what? SPD2013 isn’t compatible with SharePoint 2010. So I reinstalled SPD 2010 but every time I click the Open Site button, it crashes.
Luckily, I still had the Recent Sites list so I could still use the tool, I just had to start at the root site and navigate my way down. After a month of “dealing with it”, I asked Google for an answer. Luckily, someone else already went through the tough work of diagnosing the problem and offering a solution. His case is the reverse of mine (he probably didn’t uninstall SPD2010 first) but the resolution is the same.
Delete the ClientGUID keys (both of them) to resolve the issue.