I’m pretty sure this error only began happening after upgrading a site to 1.6. A member group entitled “owner” is now getting this error when trying to edit an entry.
In “CP Home › Admin › Members and Groups › Member Groups › Edit Member Group › Owner” and the “Weblog Assignment” option, the groups has “Yes” for all weblogs in “Can post and edit entries in:” and in “Stack Posting Privileges” everything is set to “Yes”.
Any ideas? I seem to remember there’s a “let others edit an entry not created by them?” option somewhere but can’t remember where it is. Perhaps that got changed back to “No”?