Between May 28 and June 1st, some assets could not be deleted despite being marked as not used in the asset list. This issue is fixed now and unused assets can now be deleted.
The reason is an internal change to how referenced assets within any setups are managed. Before May 28, virtual playlists where created for each setup. These playlists referenced assets used within a setup. Deleting an assets wasn’t possible if such a reference existed. After May 28, this mechanism now works in a similar way but data is stored slightly more optimized. Assets referenced through the new mechanisms also could not be deleted.
The problem was that the old playlists where not deleted during the May 28 migration. The old playlists still existed and held references to used assets as of May 28. Any assets not used after that date were marked as deleteable as no reference exists in the new mechanism. But actually deleting such an asset was then prevented by the old reference within the now obsolete playlists. Removing the old and obsolete playlists fixed this issue.