I have encountered the same problem with both administrator accounts on the same installation. Existing pages are off-limits for administrators, as "You are not allowed to edit this page" appears in lieu of the editing fields.
Additionally, administrators have the ability to post new pages, but not to publish them - a [Submit for Review] button replaces the [Publish] button; new pages cannot clear a review process if the administrators don't have the capability to do so.
Lastly, administrators lack the ability to delete pages; every attempt is met with a "You are not allowed to delete this page" alert.
The installation in question has a database prefix of wp_xxxxxxx_, and the fix given above does not seem to have any positive or negative effect. Another contributing factor to this headache may be the non-linear upgrade that was made from 2.0.x to 2.6.0.
Plugins have been deactivated with no effect on the lack of page editing capabilities. Under the hood, the wp_xxxxxxx_user_meta information checks out - both administrators have the correct data assigned to them.