(Kind of amazed that I today can't find anything when search on Preparing to move [to] … Can't believe I'm the only one having this problem. )
Been using Dropbox since 2010 and it has had it's ups and downs and definitely feel it has worsened user experience since Dropbox changed to Apples file-provider services in 2022-2023.
Saying this as I have very extensive experience of using Dropbox and to the larger degree, quite positive experiences.
But, since a few weeks ago when I updated my system to macOS 26.4, I've started getting "Preparing to move to"… dialogs very frequently. And dialogs that stays up for seconds, sometimes quite a few.
Just run the problem in ChatGTP which has lots of explanations including recommendation to move files in terminal…. Not tried anything yet.
Posting this here to see what others has come up with. And hopefully also some comments from Dropbox the company itself.