In order to reproduce this issue, I also had to be working within a folder that was syncing with dropbox. Further, I could never make this issue happen with JPGs, so I tried it with a PNG as shown in your screenshot. At that point, I had a 173 KB PNG file, and reproducing this issue required me to repeatedly spam the T key to tag and untag the image, which would happen successfully several times before the error occurred.
Then, I tried with a larger PNG of about 3 MB, and the error happened much more frequently, however I still needed to press the T key very rapidly in order to get this message to appear. I don't know if file size has to do with this, but if Dropbox needs to lockdown the file until it can sync the changes, I assume file size could play a part in this since a long file would likely take longer to sync. However, if Dropbox is locking down the file at random times, I'm not sure if there's anything we can do about this.