SharePoint can do some pretty weird stuff. One day everyone is publishing quite happily. The next, they cannot add a new page, and some page properties give errors. They are normally presented with an "Error: Access Denied" page even though they are presented with the link to create a new page. While we have not traced what causes this, we have been able to fix it. The problem lies with Page Layouts and Style Libraries.
To resolve the issue, you will first have to log into the site as a site owner. Go to the top site and then Site Actions, All Site Settings, and select Master pages and page layouts. Go to Document Library Settings and click Permissions for this document library. Now add the group that you want to be able to create pages to this list and select at least "Restricted Read."
Now go back to your top site and select "View All Site Content" and select "Style Library." Once again go to Document Library Settings and click Permissions for this document library. Again add the group that you want to be able to create pages to this list and select at least "Restricted Read."
Your users will once again be able to create pages. Keep an eye on this, because we have suspicions that there is some automated process that clears the permissions for these libraries, possibly an update.