If you see this kind of error while opening aspx pages from SharePoint Designer 2010, following troubleshooting should work.
Open Central Administration
Go to: Application Management > Manage Web Applications
Click on your web app > General Settings dropdown, select “SPD Settings” and Make sure all are enabled.
Open your Site Collection
Go to: Site Actions > Site Settings > Site Collection Administration > SPD Settings
Make sure all are enabled.
However, In my case SPD uninstall and re-install worked. J