I'm not sure what settings we changed, but I can no longer view a Web Part Page in the SP Designer "Design" window/view. I receive the same warning statements if I create a new Web Part Page (but ASPX/HTML Pages work fine) or try to open an old Web Part Page.
When I open a web part page in SP 2010 Designer the window appears with: "You do not have permission to do this operation. Ask your web site Administrator to change your permissions and then try again, or log on with a user account that has this permission. To log on with a different account click okay."
When I click cancel the following appears in the "Design" window. Master Page Error The Master Page http://xxxxxx/_catalogs/masterpagev4.master cannot be loaded. Attach a different Master Page, or correct the problem in code view.
1) Previously, I was able to edit/create Web Part Pages without any problems. 2) I can use "Advanced Mode" and alter the code, but still receive the same Permission Error shown above and still cannot see the page in the "Design" Window. 3) When I check permissions on a Page, I am listed as "Owner" - Full Control, Limited Access. 4) Central Admin - Sharepoint Designer Settings: All 4 boxes are checked. 5) I can still create/edit workflows and edit pages outside of SP Designer.
If you have any idea of what I changed to prevent viewing/editing in the SP2010 Designer "Design" window I'd really appreciate any help.