A one-off issue with read-only

I’m on MacOS 10.13.3, and I’m using Scrivener 3.0.1 on my MacBook. The drive is 500G SSD.

And, I am a single user; my setup includes DropBox, which was setup precisely as described by L&L and which util today worked flawlessly on both my Mac and on my iPhone. On the iOS side, I am running 11.2.6. I do use iCloud. My Scrivener file path is: /Apps/Scrivener/MyFile. Pretty conventional and very dependable until today.

When I opened Scrivener today I learned that

This happens when opening through Scrivener, or double-clicking from the Finder. The odd thing though is that I do have a previous iteration of the same file living in the same path; opening that previous file via Scrivener or via double-click is successful.

Given this behavior, to me I could see little sense to make permission changes, but I have no idea what is happening.