Just upgraded a site from 1.6.6 to 2.5.5 and when I went to edit an entry or publish a new entry I got
Unable to load the requested file: content/_assets/glossary_items.php
From initial digging on EE forums, they all pointed toward Matrix being the issue, however this site had ZERO add-ons.
I installed Matrix and the issue is now non-existent. Having Matrix installed isn't really a solution.