That looks like Dropbox is returning the 409 error but without seeing a small chunk of your workflow it is hard to say where the issue is, I suspect it is an issue with the workflow itself rather than an issue with Dropbox or n8n.
I have since evaluated the issue further. Its seems that there is something odd happening when it comes to dropbox. Typically dropbox auto organizes files in sub folders, therefore at certain times the path can have an issue because it has already posted the file to a particular folder and the path at times is incorrect. I am suspecting this is the main cause. I have since contacted Dropbox Support, I am not sure how fixable this is unless I turn off the auto file organization off. This means all my files will be scattered all over, which is not ideal. I have setup an error handling scenario to fix this issue. We should be ok but something drop box should look into. Their API is also a hit and miss have noticed.