Magento : 404 page not found when trying to access CMS -> Pages in admin panel

Some leftover CMS-Page database entries were referencing a store_id of a store that no longer existed after I deleted it.

So here's what solved it for me:

Find out the ids of your stores: Under System > Manage Stores in the column Store View Name look for the Store's id that pop up when hovering the link and also in the link url
(e.g. ...admin/system_store/editStore/store_id/3/key/...)

To be safe, make a database backup under System > Tools > Backups

Go to your database and find the table cms_page_store. Delete all entries with a store id in the field store_id that points to a store, that does no longer exist.

Refresh CMS > Pages, 404 should be gone.