This thread already starts to address the issue, but it never reports if a solution was found or implemented:
https://www.dropboxforum.com/t5/Create-upload-and-share/Dropbox-asks-if-I-want-to-remove-a-document-right-after-I-save/td-p/388342/page/6
I am having the same issue. In my case, it seems to be related to firefox profiles I am saving to Dropbox.
I am not 100% sure, but I believe the culprit folder is:
C:\Users\"User1"\Dropbox\FIREFOX PROFILES\SHOPPING\cache2\doomed
The "User1" would be replaced by your own machines username.

Windows 11.
Firefox 110.0 (64-bit)
Feel free to ask for more info if required
Regards