Paul, one has to make sure, that the displayed_object_id is passed properly from the application to the toplevel template, such that the ckeditor knows to which object it should attach images. Although the displayed_object_id is an old thing, it is/was not passed properly in several templates via "property" statements. Since the permissions depend on the displayed_object_id, my first guess is, that this might be the problem in your installation.
The functionality of handling images in ckeditor via attachments was added after the 5.9.1 release, so make sure, you have the changes for the theme [1], for news [2], forums [3], or xowiki [4]. Check as well your custom templates if applicable, whether these pass a property for "displayed_object_id". Note, that xowiki can handle embedded images also via the xowikiimage plugin, which handles images as subobjects of wiki pages and which uses xowiki image markup (with "[[image:...]]"). In the latter approach, pages with embedded images can be exported/imported in other OpenACS instances, but with the ckfinder/attachments approach, images are currently not preserved.
Concerning file-storage dependencies of the attachment package: The attachment package has essentially 2 parts: (a) an internal API and (b) a user interface. The way the attachments are used in the ckeditor packages, only (a) is used, since the user-interface is provided by the ckeditor4. The part of the attachment-description is based on the assumption, that everything works over the file-storage.... it should be made more precise.
hope this helps
-g
[1] https://github.com/openacs/openacs-bootstrap3-theme/commit/3de9001142a3efb9224c7c1859e3bcc6d89800c2
[2] https://github.com/openacs/news/commit/cc328c52422c9dae00641bdca4c7412a0dc973c3
[3] https://github.com/openacs/forums/commit/fbaeb7827ba25ac7a37229a26e1c076f4adfd603
[4] https://github.com/openacs/xowiki/commit/a1627866bfb72cd6b923a9855b7168b5c6f3d2ca
