I am on the latest iOS and Dropbox for my iPhone 13. Dropbox 324.2 and iOS 16.4.1
I noticed that it was going forth between 5 and 4 photos left. There is plenty of room in dropbox. Nothing has been uploaded.
I followed the instructions mentioned here https://www.dropboxforum.com/t5/Storage-Space/Photo-backup-on-iPhone-isn-t-working/td-p/515471 for clearing the cache and rebooting. Still have the same issue and it won't give it up.
Problem also exists backing up via cellular.
Problem also persists after full delete and reinstallation of Dropbox on iPhone.