After sites migrated from SharePoint 2007 to 2010, you may find some site owners with full permission could not edit the default page. The error is as below.
We had about half dozen sites that were having such issue after 2010 upgrade in August. We were able to track down the problem keeping our “Full Control” users from editing pages. The problem is a lack of permissions on the associated site libraries after the migration. Specifically, the Master Pages library. We were not be able to identify how but here is the solution to fix it.
Go to Site Actions –> Site Settings -> Master pages and page layouts -> “Library” tab from ribbon -> “Library Permissions” button at the top, right ->
Add the users or group to permissions here with at least Read access enabled.
Now, owner groups will have the permission to edit the pages.